It is vote that could divide one island - but re-unite another.

That, at least, is how our independence referendum looks like to many unionists on either side of the North Channel: an existential threat; a potential extinction event.

Break up the UK, the theory goes, and there is nothing for loyalists to be loyal to. Ulster can't be British if there is no Britain.

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A simplification? Yes. But one so potent it already has politicians in Dublin very worried indeed. Just as worried, in fact, as some of their counterparts in Belfast.

This week The Institute for International and European Affairs, Ireland's answer to Chatham House, published a report called "Scotland's vote on Independence, The Implications for Ireland."

Author Paul Gillespie, a former foreign editor of the Irish Times and an authority on UK-Eire relations, spelled out that Dublin angst on the indyref.

Irish policy makers, he said, are watching Scotland's referendum "with growing fascination and alarm".

"If the UK breaks up," he wrote, "then Irish unity would be put on the political agenda far more quickly than Irish political elites and voters north or south expect or desire.

"This is not to predict that outcome, but it is to say it would become a more urgent political option than now, with potentially rapidly changing preferences on all sides, including among unionists who would lose the object of their traditional loyalty."

So why would Ireland worry about re-unification?

The Republic, bluntly, suggests Gillespie, wouldn't fancy footing the £10bn a year bill to run the north. Not right now, anyway. "Northern Ireland," he said, "would be a daunting task to absorb for an Ireland coming out of the EU/IMF programme."

The same financial concerns explain much of the concern about Scottish independence among some unionist politicians in Stormont. They see any rocking of the constitutional boat as a threat to their fiscal lifeline to London.

Cannier observers reckon that Northern Ireland has found a kind of strained but working equilibrium in recent years. So too have relations between Ireland and the UK.

Both those balances, warned Gillespie, could be undone by two plebiscites, the indyref and David Cameron's promised "in-out" vote on membership of the European Union.

Gillespie spells out the four possible scenarios for the "British" Isles as a consequences of those two votes.

They are, or should be, familiar to all of us. But lets list them anyway:

* Scotland stays in the UK and the UK stays in EU;

* Scotland stays in the UK and the UK leaves EU;

* Scotland leaves the UK and the rump UK stays in EU;

* Scotland leaves the UK and the rump UK leaves the EU.

So what might these different possible futures bring? Gillespie has thought through each of them. His conclusions:

Scenario 1: Even if Scotland stays in UK and the UK stays in Europe, Britain would edge towards federalism, provoking a new British-Irish relationship. This is the most likely outcome of all, Gillespie believes.

Scenario 2. If UK quits Europe because of English conservatives, the Scottish question would re-open even if Scots had voted to remain in UK. "Major issues would be posed for the British-Irish regime," explained Gillespie, " including that the Irish border would become the EU one, potentially creating a messy and more harshly competitive regulatory environment."

Scenario 3 Scotland leaving the UK and the UK staying in the EU would also mean a rethinking of UK-Irish relations as the rump Britain became more Anglo-centric. This scenario, said Gillespie, "would unsettle the UK and reduce its solidarity because a dominant and increasingly less communitarian England would be less willing to share power equitably with Wales and Northern Ireland, despite their probably desire to remain part of the union". Gillespie sees no problem on Scotland getting back in to the EU - "a pragmatic agreement would be reached," he said.

Scenario 4. Scotland votes Yes to independence and then rump Britain decides to quit Europe is the scariest prospect for Ireland. "A UK break-up is likely in this case," argues Gillespie, "since predominantly Eurosceptic England with fewer communitarian or solidaristic values would increasingly resent and be less willing to fund Wales and Northern Ireland; they too would want to rethink their futures despite their existing distinct but deep relations with the rest of the UK."

It could also, warns, Gillespie, mean a "radical change in the British-Irish regime of complex interdependence" cemented, uniquely, in an international treaty, the Good Friday Agreement.

"Irish leaders, says Gillespie,"know how risky referendums are"; they have lost a few. Gillespie says the Republic's government has accepted Cameron's promise of an "in-out" vote on the EU "as a regrettable political reality".

He added: "The rapid growth of Euroscepticism, intimately associated with the English nationalism of UKIP, is forcing a transformation of the UK's relations with the EU….

"The prospect of a UK withdrawal is alarming for Irish policy-makers because it would jeopardise key conditions that have brought Britain and Ireland closer together over the last generation."

But Gillespie also sees any rise in anti-EU English nationalism as a crucial element in the Scottish vote itself.

He said: "A strong UKIP showing in the 2014 European Parliament elections would sharply remind the Scots that the referendum will also be about their co-existence in a continuing union with a much more Eurposceptic and Conservative England."

Alternatively, he said, Labour doing well in the Euro polls could reassure Scots.

He added: "The key variable here would be Conservative Euroscepticism as a source of Othering in the development of Scottish political identities - as is typical of such nationalist movements elsewhere - given the toxicity of contemporary Conservatism in Scottish political culture, whose communitarian centre of gravity is to the left of English individualism."

The prospects of Scottish independence for Ireland aren't all bad. Far from it. In fact, Gillespie believes that an independent Scotland could be good for Ireland if rump UK stays in the EU

He said: "This outcome might suit Ireland's interests best, as Scotland could become an Irish ally in the new EU setting.

"However, it could also see Scotland become a determined competitor for investment as a much as a Celtic soul-sister."

I've heard this last view whispered from Ireland too. Irish policy-makers like the idea of another medium-sized state emerging with similar strategic outlooks to their own. But, privately, they do fret at the prospect of a Scottish competitor. "Suddenly we wouldn't be the only ones who could turn on the Celtic charm in Washington," one Irish commentator told me.

Gillespie has one last thought I wanted to share with you: that working class Scots with Irish heritage, especially in the west, could swing the indyref one way or another.

A Yes vote, especially if combined with a rUK exit from the EU, would have a huge impact on Irish-re-unification. But could the prospect of Irish re-unification have an impact on a Yes vote? Will Irish unity be in thoughts of any Scots, even with links across the North Channel, when they cast their ballots? Probably not. But I could be wrong. So what do you think?