ON Wednesday night, Glasgow's Royal Concert Hall hosts a performance by musician Carlos Nunez. Nunez plays the bagpipes, by the way, and, so the programme notes tell us, hails from Galicia. Ordinarily we'd call him Spanish, of course, but not here because Wednesday is the opening night of the annual Celtic Connections festival. And the difference matters. For these few weeks in January, countries . . . at least the sort that can issue banknotes and passports . . .

become an irrelevance. They melt into the background.

Spain, France, Portugal, Canada, Wales, even Scotland: just names on a map now.

Instead, superimposed on top of them is a deeper cultural identity, one which unifies these Gaels, Gauls, Galicians, Asturians, Cornishmen, Manxmen and Bretons under one umbrella term. The clue's in the title: Celtic.

At least that's the theory. In practice, that umbrella affords little shelter. There are too many questions crowding under it begging answers. For a start, what do we mean by Celtic? Secondly, in the context of our own country, are all Scots Celts? Is there a Celtic nation within our Scottish one and if there is where is it and how does it - how can it? - find political or philosophical expression? And as the Celtic brand sweeps all before it, what do we gain by throwing every Scottish cultural artefact into its embrace?

Perhaps more importantly, what do we lose?

I'm setting off in search of the Celtic nation. It may exist in pockets, it may exist only in song and myth. In fact I'm not sure that the terms "Celtic" and "Scottish" aren't already indistinguishable. But if they are not then my journey is an attempt to tease out the difference between the two and determine whether Celticism is, as some argue, a vital cultural force and one which can shape in this century the new Scotland we glimpsed at the end of the last. Chimera or reality? Let's see . . .

I start in Glasgow, perhaps Scotland's most Celtic city if by that we mean the one which most embodies the traditional Celtic attributes: vivacity, loquaciousness, a love of music, drinking and fighting.

Colin Hynd is director of the Celtic Connections festival and has seen it go from strength to strength since its 1993 launch.

He is well aware of the growing power of his brand but, 13 years on, he is no closer to a good definition of the word Celtic.

"When we started I spent a long time talking to people about what it meant and I never found two people who agreed, so it's quite a vague concept in my mind, " he says.

"But I primarily link it to music and musical culture and the connections between places like Galicia, Asturia, Britain and Ireland."

Many miles north of Glasgow is Ullapool where Jean Urquhart runs the Ceilidh Place, a well-established live music venue. She agrees with Hynd's loose definition but she is also prepared to put borders on this Celtic nation. For her, Celtic Scotland is confined to the western half of the country and runs from Dumfries and Galloway in the south, right the way up the coast, taking in the historic Celtic stronghold of Argyll (the ancient kingdom of Dalriada) and, of course, the Western Isles.

"My own definition of Celtic would be the Atlantic rim countries, " she says. By that she means Scotland, Ireland and Wales as well as the Isle of Man, Brittany and Galicia.

"Those areas do all seem to have strong links with each other. Within Scotland I see it as a west of Scotland thing. However I recognise that the whole of Scotland is seen as a Celtic nation and I'm perfectly content with that."

But is everyone else equally content? I travel to Stirling, to the Wallace Monument.

Here we commemorate William Wallace's victory over the English invader Edward I at the Battle of Stirling Bridge in 1297.

Nearby is Bannockburn, where Robert Bruce defeated Edward II in 1314 and set about the process of nation building that resulted in the Declaration of Arbroath in 1320. The modern Scottish nation could be said to have been forged in this place.

In the car park, by the visitor centre, is a statue hewn from stone in 1996 by local mason Tom Church. It purports to be of William Wallace but in fact it shows Mel Gibson as that character in the film Braveheart. The word is even emblazoned on his shield. He's also wearing what looks like a kilt. Since the garment was associated at the time with the heathen Highlanders, Wallace wouldn't have been seen dead in such an item, but here he looks every inch the Celtic warrior. I watch as a father uses his camera phone to film his two children jumping into the air beside the statue. As they leap, they shout the single word chiselled into its base: freedom. The kids fluff it first time. Everyone laughs. Dad makes them do it again. It's a wrap. The kids look proud.

I collar another tourist, Alex Bowes. Does he feel Celtic? "Do you mean Celtic or Celtic [the football team]?" he asks. I've been waiting for that one. "Because they're spelt the same a lot of people think Celtic means a Catholic tradition and I think that's wrong.

People that support Celtic FC actually think it's a Catholic tradition. I don't feel Celtic.

I'm Scottish. British actually. I'm from the United Kingdom."

Alex is from Alloa and supports Alloa Athletic, though he will admit to a penchant for Rangers if push comes to shove. Does that make a difference? Is Celticism antipathetic to Unionism or Protestantism? In a sense it is:

our most recent manifestation of the Celtic warriors of old are the Jacobite armies who backed the Catholic Charles Edward Stuart against the Hanoverian monarchs. Then again, as I will learn later in my journey, one of the characteristics of the ultra-Protestant Covenanters was a strict religious devoutness, itself a well recorded Celtic trait.

In Stirling city centre, on Port Street, I meet Alistair Tripney, a piper with the Argyll and Sutherland Highlanders. He's tuning up outside an army recruitment office preparing to play, but his commanding officer agrees to let him talk.

"To me, Celticism symbolises Scottish traditions, " he says. "First and foremost in terms of music, because I'm a musician.

But I think it's become a general term for all the Scottish traditions. There are different traditions from the east coast, the west coast, the Highlands and the Islands.

Everyone has different traditions but in my opinion calling them Celtic brings them all together. It works for most kinds of music, especially the traditional stuff."

Tall, laconic, with brown hair and piercing blue eyes, Tripney is from Kilsyth.

He doesn't look like a Celt as I imagine them - small, dark, fiery - and, despite what he's just said, doesn't feel like one. "I'd class myself as Scottish, " he laughs. "I'm not sure if I'd call myself a Celt."

In the local tourist office I meet Elizabeth Angus. For her, Celticism is as much about language as it is location.

"To me it's something which has a Gaelic content, which not everything has. It would have to be connected to a Celtic language, though I think that has altered somewhat now."

Her own Celtic connection she describes as "tenuous" - there are some Gaels in the family - and, like the others, she thinks of herself as Scottish rather than Celtic. Asked whether she thinks Scotland can truly be described as a Celtic nation, she pauses for a beat or two. "It's a difficult one, " she says finally. "I see a great divide between the Lowlands and the Highlands."

The picture so far, then, is of two nations, Scottish and Celtic. But where does the Celtic one lie? Elizabeth Angus thinks its modern heart can be found in Glasgow in January at Celtic Connections. But come February it too disappears, like Brigadoon, into the mist.

It has a commercial heart though - ironically in the most tourist-choked thoroughfare of Scotland's most English city, the High Street of Edinburgh. In shops with names like Thistle Do Nicely and Heritage Of Scotland, Celtic rings and brooches nestle alongside See You Jimmy hats and tartan scarves. In The Scotland Shop just off the High Street, a Sikh man with a Geordie accent will sell you a letter-opener on which is carved a Celtic cross or, for not a little less, a pair of knickers with "Scottish Pussy" emblazoned on them.

IN Wha's Like Us? I meet Amy Kerr.

"It's just basic souvenir stuff mainly, " she says as she surveys her stock. The Celtic jewellery is among the biggest sellers and the purchasers are mostly American, Canadian, Australian or New Zealanders, with a sprinkling of Europeans on an easyJet weekender. Do tourists differentiate between Celtic and Scottish? "I don't think people know the difference, " says Kerr. "We have to explain what the Celtic designs mean."

If the Celtic nation has a spiritual heart it is in Iona, where the Irishman St Columba set up shop and from where Scotland's slow conversion to Christianity began. It is Iona, of course, that is synonymous with the Celtic crosses which are the most potent expression of Celtic art and which are recreated for the tourists in shops like Wha's Like Us? And yet it is Andrew, a Mediterranean saint, who has become Scotland's symbolic figurehead.

That point isn't lost on Dr David Caldwell, Keeper of Scotland and Europe at the National Museum of Scotland in Edinburgh.

He walks me round his domain, just a few hundred metres from the High Street, and we loiter by the famous Lewis Chessmen to talk. For many years Scotland turned its back on its Celtic past, he says, though that is changing. He sees the kilt as instrumental in this process.

"When Scots dress up in kilts and look at things like bagpipes they see it as being part of their Celtic background, " he says. "One of the things that intrigues me is that since the late 20th century there has been much more acceptance of tartanry. More and more people now expect to get dressed up in tartan."

SNP leader Alex Salmond made the same point recently in a review of Allan Massie's The Thistle And The Rose, a history of the long and fractious relationship between England and Scotland. Tartan, he opined, has been rescued from the ghetto in which Sir Walter Scott left it. It has been re-invented by fashion designers, accepted by youth culture and even used as a uniform by the Tartan Army, followers of Scotland's benighted football team. "I attended a school dance in my constituency last year, " he continued. "Every lad was sporting tartan of varying degrees of outlandishness. If I had tried that at Linlithgow Academy in the 1970s I would have been beaten up in the toilets."

Perhaps, then, the Celtic nation is actually a Kiltic nation and can be found at every wedding, birthday party, Hogmanay bash and international football match at which tartan is worn. It is then, a ceremonial nation.

The historian Professor Tom Devine has thought long and hard about kilts, Celts, Scottishness, nationhood and how and where they interface. It was Devine who last week issued a stern rejoinder to historian Professor Niall Ferguson's statement that Scotland should just become part of England.

What I call Celticism, Devine calls Highlandism, preferring that term because he sees in the image of the Jacobite Highlander the last expression of a politically active Celtic culture.

At the time of Culloden in 1745, the Highlanders were viewed as barbaric, heathen and - even worse - Popish. They were defeated, of course, and Scotland went on with the business of being a stateless nation prospering under the union with England. For Devine, these two facts are crucial for what happened next: the re-invention of the Highlander, and therefore the Celt, as a romantic figure.

"Scotland could not evolve a wholly or an aggressively nationalist ideology in the 19th century because it was doing so well out of the Empire. But at the same time, spiritually and emotionally it was a nation - a stateless nation - and it had to shape an identity which was politically harmless, which would fit in with the union, which would not threaten it but would be identifiably Scottish, " says Devine.

This process was in train before Sir Walter Scott put pen to paper, but today he is seen as its wellspring.

Heroic and tartan-clad, Scott's novels sculpted a mythical nation out of a country which was fast becoming the world's most industrialised. As the cities boomed, our hearts were in the Highlands, soon to be joined by monarchs like Queen Victoria. In 1820, the year before George IV arrived in Edinburgh for a state visit wearing a kilt, a Celtic society was founded in Edinburgh and in 1838 the first Celtic Congress was held in Wales.

Cleverly, the British army gave all this a human face by dressing its Highland regiments in tartan. That way the image of the Scot as Celtic warrior persisted, even if the warriors in question had actually been co-opted into the Imperial project and were often to be found fighting in its farthest dominions.

Today, says Devine, Celticism is being reinvented in tandem with a Scottish identity which is becoming increasingly more robust.

"The current statistics suggest that two thirds of sampled Scots feel exclusively or mainly Scottish. An ethnic group like that tends to look for markers of identity and one of the traditional ones is Celticism, or Highlandism. But it's being further developed."

Part of that development is in the way Scotland is marketed and branded. It's easier to sell a unified Celtic nation of glens and tartan than one divided east to west.

It's easier to make Celticism a broad church and bring into its folds cultural artefacts which don't strictly belong there.

"What's happened is that although the Highlands were torn apart by clearance, famine and mass emigration, they had the last laugh because they became the source of a lot of modern Scottish identity, " says Devine. "Look at Visit Scotland - it's a classic example of the victory of Highlandism."

Ironically, the most comprehensive and inclusive view of Celticism I find - and one which echoes Devine's prognosis - comes from the Borders, not an area we associate with the Celts. Ah, says historian and broadcaster Alistair Moffat, a Selkirk resident, there you'd be wrong.

HIS book The Sea Kingdoms sets out to define Celticism. It urges its readers to reconsider Scotland in light of its Celtic past and use that knowledge to view Britain from a different angle entirely.

"We think of Scotland's Celts as being Gaels and Scotland's Celticism as being connected to the Gaelic language, but it's not true, " he says. "The Celtic culture of Old Welsh washed over Britain from Cornwall to Caithness and the people in the south of Scotland spoke a Celtic language well into the historic period. Celticism is a British phenomenon as much as a Scottish one.

"I think the more that people get involved with Celticism and feel involved with it the better. What it means to me is British but not English."

When we talk, Moffat has just returned from spending Hogmanay in Italy. He wore his kilt on the night and once again the camera phones came out to record the event.

He found, though, that even the Italians want a bit of the Celtic action. Some that he spoke to said they were proud to be Celts.

That strikes me as odd given their rich Roman inheritance. But then I recall that Italian footballer Paulo Di Canio, a selfconfessed fascist, was recently slated by the Italian authorities for giving a straightarmed "Roman" salute to the famously right-wing fans of his club, Lazio. Are some ancient tribes and cultures more politically correct than others? Perhaps. Certainly we shiver with embarrassment when Teutonic or Roman virtues are extolled because of the perceived connections with Hitler or Mussolini. Similarly when fans of Rangers FC are nicknamed Huns by their rivals across the city the imputation is of philistinism and, in an odd way, Englishness. Yet, despite its bad press in the 18th century, Celticism brings only Asterix to mind today.

Indomitable, fierce, eloquent, rebellious, long-haired, bardic - the Celtic brand is up there with Harley-Davidson and Levi's in the pantheon of greats.

Moffat certainly thinks of himself as a Celt. Moreover, he sees Celticism evident in Scotland still, while aspects of the Celtic personality which were stifled as Scotland became England's first colony are now reasserting themselves.

"Celtic eloquence has long been damned as blarney, generally speaking, and it's a very good example of English culture converting a Celtic virtue into a vice. What I think is happening is that those alleged vices will become virtues once again.

Confidence in who we are is the great prize.

We always knew we were Celts, we just didn't value it."

Perhaps, then, a new Celticism is being formed in the crucible of 21st century Scotland: in our parliament with its circular debating chamber, itself a call to oratory; at Celtic Connections in Glasgow in January, a call to song; in the Highland towns enjoying renewed economic growth; and in the hands of the designers re-cutting the tartan cloth for the demands and tastes of the iPod generation.

Most national identities are founded in fakery, embroidered with threads of tradition both mythical and real. This will be no different. Its greatest challenge will be to reach out to those not born on these shores, those who quite happily call themselves Pakistani-Scots or Italian-Scots but for whom the adoption of an identity with racial rather than national characteristics may be problematic. But if this New Celticism can broaden and become allencompassing - while retaining as its cornerstones confidence, talent, vibrancy and that sense of otherness which has given the Celts of old their delicious outlaw sheen - then it won't matter a jot.

Perhaps it will start on Wednesday.

Sue Wilson's guide to Celtic Connections:

pages eight and nine