TWO years ago, Jim McGuinness was the revolutionary whom Gaelic football didn't want.

The game was fine the way it was. It didn't need a giddy rebellion in the rolling hills of Ireland's north-west.

Gaelic football's aristocracy in the south wheezed every time McGuinness's Donegal team took to the field last year. Donegal were outcasts who had disobeyed the game's orthodox teachings. They were pilloried for playing a uniquely calibrated defensive system of football.

Donegal were one big turn-off. Of course, successful teams that went before McGuinness had been innovative; forward-thinking coaches began viewing the game from a different angle. In the last decade, some coaches placed greater emphasis on defending. Forwards began shuttling back into defensive positions – but nobody placed as many players in defensive areas as McGuinness.

In their 2011 All-Ireland semi-final defeat by Dublin – it ended 0-8 to 0-6, a desperately austere, low-scoring encounter in Gaelic football terms – Donegal, for long periods, played all but one of their 15 starting players in defensive areas. The face of Gaelic football had changed forever.

There was outrage among the GAA punditry. The game was no longer about individual duels in designated areas of the field. The new version of Gaelic football, practised by McGuinness, had morphed into a puzzle about space and how best to utilise it.

There are thinkers of the game – and there is Jim McGuinness. In his first year, he was lambasted for playing negative football. In 2012, he was lauded for transforming Donegal into the most devastating counter-attacking team in the country, and undoubtedly the fittest there has been.

To understand Donegal's defensive tactics, you first had to understand Donegal. They hadn't won a meaningful game in three years before McGuinness took over. "It was like a dog that had been battered," he said. "You call it and it won't come to you. That was the kind of situation we were in."

Before the former county player assumed the reins, the Donegal players had nurtured an unflattering image of being the party boys of Gaelic football, a talented crew, but never keen on putting the hard yards in. That would change irreversibly under McGuinness.

The Glenties clubman was destined to be a coach. "I started coaching when I was about 16 with under-12 teams at the [Glenties] club," he explained. "It was always something that I enjoyed doing."

During his playing days, McGuinness was full of flair. Back then, there was spontaneity about Gaelic football. The game was an entirely different spectacle.

McGuinness was part of the Donegal side that last won the All-Ireland title in 1992. Ironically, McGuinness the player and McGuinness the manager reside at different ends of the spectrum.

Throughout his playing career he was an engaging, heart-on-the-sleeve personality for the media. Armed with a diploma in sports studies and a degree and a masters in sports psychology, he had applied for the Donegal job three times before he was successful, but served an invaluable apprenticeship with the county's under-21 side.

When he took over, Donegal may not have been the most charismatic of sides, but their manager had bags of the stuff. McGuinness doesn't walk into a room. He swoons. He can work a room of journalists with the best of them.

Anyone who meets the 39-year-old will be impressed. He is articulate, engaging, sincere and at times funny; but there can be a sharp, unforgiving side too. At the end of the 2011 campaign, McGuinness controversially axed one of his key players, Kevin Cassidy, for revealing minor details of Donegal's new regime in a local GAA book.

While Cassidy was exceptionally generous in his praise of the new manager in the book, McGuinness nevertheless deemed the player's actions as a breach of trust. Cassidy was gone. But it didn't end there.

Moments after he had guided Donegal to an unlikely All-Ireland title in September, McGuinness refused to carry out his post-match press conference until the book's author, also a working journalist, was ejected from the room. It was an unseemly and desperately ill-timed episode.

McGuinness shares a nerdish love for many team sports. It's been a long-held ambition of his to become involved in professional sport. He didn't have a day job while managing Donegal last year.

Nobody on this side of the Irish Sea expects McGuinness to be anything other than a resounding success as Celtic's new performance analyst. There is, however, the nagging feeling that perhaps he should relinquish the Donegal reins.

"Rather than try to hold on to the Donegal job," wrote Armagh's All-Ireland winner turned respected pundit, Oisin McConville, "he should walk away . . . I don't think it is possible to serve two masters".

McGuinness insists he won't miss one Donegal training session in 2013 and that his Celtic role is initially a "couple of days a week". Knowing McGuinness's meticulous nature, he will probably have already enrolled in a time management crash-course.

In McGuinness, the Scottish champions have hired a radical thinker and an immensely likeable individual who will do whatever it takes to succeed in his chosen field. Celtic's academy players will not have realised it yet, but they couldn't have asked for a better mentor than the Glenties native.