Last Friday, British prime minister Theresa May went to Glasgow to affirm fidelity to an idea that has seldom looked shakier.
Rejecting Scottish demands for greater autonomy after Brexit, May said she would fight against any further transfer of powers from London if it meant the United Kingdom would become "a looser and weaker union".
“We cannot allow our United Kingdom to drift apart,” she said.
London's concerns about the future of the union centre on Scotland, where the drive for independence, its momentum stalled by an unsuccessful referendum in 2014, has been revived by last June's decision by UK voters, against the will of a majority in Scotland, to leave the European Union.
Given what was unfolding in Belfast, however, perhaps May should have gone there instead.
That day, as she was addressing the Scottish Conservatives in Glasgow, ballots were being counted after an Assembly election that had drawn so little attention across the Irish Sea that it might as well have been taking place in Namibia as in Northern Ireland.
Unionists are spooked. The day they thought would never come has suddenly come and come very unexpectedly.
The result, which put Sinn Féin within a single seat of replacing the DUP as the biggest party and deprived unionism of a majority for the first time, sent a tremor through northern politics.
“Nobody in our system or the British system believed it was in any way likely that Sinn Féin would come so close to overtaking the DUP,” says a senior Irish official.
"Unionists are spooked," says Alex Kane, a political commentator and former director of communications for the Ulster Unionist Party. "The day they thought would never come has suddenly come, and come very unexpectedly."
Relatively prosaic political factors, chief among them DUP leader Arlene Foster’s ability to mobilise the nationalist vote, partly explain last week’s surprise result.
But in Dublin and London, the outcome is seen as evidence that Brexit is already upending some old assumptions and bringing long-settled questions back into play.
"I don't think people have yet woken up to the extent and depth of the anger across the whole spectrum of the nationalist community in the North over what has happened," says Brendan Halligan, founder and former chairman of the Institute for International and European Affairs, a Dublin-based think tank.
The Irish official agrees and suggests a link between the spike in the nationalist vote, after years of decline, and the Brexit shock.
“There was a psychological blow sustained by nationalists on account of what happened in the referendum,” he says.
“That comfort that northern nationalists had from being in the EU will be no more.”
Of all the relationships that will be tested by Brexit, perhaps the most sensitive is that between the two parts of Ireland.
After decades of increasing co-operation and exchange, and the gradual fading away of the Border as a feature of daily life on the island, Brexit threatens the return of a hard frontier, new barriers to trade and the psychological separation that all of that would entail.
For the two governments, the best-case scenario on Northern Ireland is a post-Brexit landscape that looks pretty much unchanged.
But Dublin and London are just two of 28 players at the negotiating table, each with their own interests and concerns, so nothing is guaranteed.
At one extreme, Brexit could drive North and South farther apart, undoing a key dimension of the Belfast Agreement.
At another, it could accelerate moves towards a united Ireland.
Alive to the tactical opportunity Brexit presents, and conscious that it speaks for the majority in Northern Ireland by having opposed withdrawal from the EU, Sinn Féin has called for a Border poll and for preparatory work on reunification to begin.
Fianna Fáil leader Micheál Martin has said he hopes Brexit will "move us towards majority support for unification", while in recent weeks Taoiseach Enda Kenny has been telling European leaders that any Brexit deal must include language that would allow Northern Ireland automatic admission to the EU in the event of a united Ireland.
He has pointed to the provisions that allowed East Germany to join West Germany and the EU seamlessly after the fall of the Berlin Wall as a precedent for Ireland.
Irish officials play down the significance of Kenny’s remarks, saying he is simply insisting on the integrity of the Belfast Agreement and seeking to reassure those who fear the British could resile from the commitment to honour a future referendum on the constitutional question in the North.
Others see in Kenny’s move an attempt to manage republican anger over Brexit.
“Talking about it [a united Ireland] maintains a certain level of stability in the North,” says Halligan.
“It helps maintain the psychological dominance of the nationalist faction that believes in constitutional action. That’s critical, because any popular support for violence only comes out of a sense of desperation that there is no alternative course of action.”
According to Kevin Meagher, special adviser to former Northern Ireland secretary of state Shaun Woodward and the author of A United Ireland: Why Unification is Inevitable and How it Will Come About, Brexit represented "an accelerant that is poured over the dry tinder of a UK constitution that is ripe for reform anyway".
He believes a British government acting rationally would acknowledge its own ambivalence towards Northern Ireland and use Brexit to begin to set out its plan for unification.
After all, he says, London long ago conceded that it had “no selfish strategic or economic interest” in Northern Ireland.
“Britain has got a mental block on Northern Ireland. We are shown no evidence that we want to retain it. It’s not meaningfully integrated at any level – politically, economically or socially – with the wider British state. It’s 2 per cent of our GDP.
"It's 1.8 million people, which is the population of Hampshire. It is kept very, very separate.
“But by the same token, British politics lacks a choreography that would lay out what we do about this. We muddle through. There is a very peculiar British hypocrisy about Northern Ireland and other territories.
“ If we were being rational, we would clear the decks and move on. But British politics doesn’t feel able to have that conversation because it fears a political backlash.”
The idea of a united Ireland may have entered mainstream discussion in recent months, but there is little sign of the attitudinal shift that would be required to make it a short- or medium-term prospect.
Under the principle of consent, enshrined in the Belfast Agreement, unification can only happen if a majority in Northern Ireland votes for it.
In a cross-Border RTÉ/BBC opinion poll in 2015, when respondents were asked if they would like to see a united Ireland in their lifetime, 30 per cent in Northern Ireland answered Yes.
The poll highlighted public ambivalence in the Republic: while 66 per cent of respondents south of the Border said they would like to see unification in their lifetime, only 31 per cent said they would be in favour if it meant paying more tax.
If religion were a reliable guide to voting intentions in a Border poll, the rising Catholic population in the North (45 per cent in 2011, compared to 48 per cent Protestant) would suggest unity was around the corner. But the correlation is actually quite tenuous.
In the 2011 census, just one-in-four people in the North described themselves as exclusively Irish, whereas 40 per cent regarded themselves as solely British and 21 per cent considered themselves as Northern Irish only.
So while the bulk of nationalists and republicans may have voted for the UK to remain in the EU, it does not necessarily follow that those voters would also opt for a united Ireland tomorrow.
“We haven’t seen a dramatic shift in voting patterns,” the Irish official says of the Assembly elections.
"Even the modest proposal made by Mike Nesbitt [the UUP leader called on his party's voters to transfer to the SDLP] seems to have produced a negative, not a positive, effect for the UUP.
“If you can’t do that, then the idea that there’s a lot of people from the unionist community gagging for Irish unity seems difficult to believe.”
Even in the event of a bad Brexit deal that had disastrous effects on Northern Ireland, would many unionists be receptive to the argument? “If it’s made cleverly,” Kane replies.
“I told a Sinn Féin audience recently: ‘you can’t win a united Ireland on the basis of 49.9 per cent to 51.1 per cent.
“ It doesn’t work like that, because you’ll end up with an awful lot of unionists deeply unhappy and some who would probably think that violence is their best option in those circumstances.’”
And while there may be a strong socio-economic case for unity post-Brexit, he adds, hardcore unionists, just like their republican counterparts, are unlikely ever to be swayed by economic arguments alone.
"I could go to places on the Shankill and say: 'look, if we joined a united Ireland in five years' time, every single one of you would be employed, your standard of living would go up and you would be better off in all these ways, and you'll have much stronger rights in the Dáil.'
“Their answer would be: ‘fuck it, Alex, I don’t care. I’m a unionist. I’m British.’ No amount of intellectual debate or economic data or rhetoric from either side is going to change those people’s minds.”
The direction of the debate around Northern Ireland’s long-term future will hinge on the terms of the deal under which the UK will leave the EU. In the immediate aftermath of the referendum last year, the call went up for “special status” for the North.
The phrase has become politically difficult – the DUP rejects any differentiation between the North and the rest of the UK, while member states such as Spain (with an eye to Catalonia) fear setting a precedent.
Sources say that, instead, the European Commission and the two governments will identify a number of key areas for Northern Ireland (the Common Travel Area, agri-food, peace and regional funding, energy and so on) and deal with them individually rather than as part of a single agreement for the North.
The aggregate result of these individual deals could amount to a “special status” but political sensitivities mean that terminology will be avoided.
There appears to be growing optimism that the Common Travel Area can be retained, although this depends on Ireland not being under pressure to join the Schengen Zone and the UK opting not to place visa requirements on other EU citizens and instead opting to control immigration through employment restrictions. A new customs regime could prove more problematic.
Dublin and London are conscious of the damage a hard Border would cause and are adamant that must be avoided.
If there has to be Border, then one option is to move it to the middle of the Irish Sea
"A Border with customs post would have security consequences – that's what would worry me," says Eunan O'Halpin, professor of contemporary Irish history at Trinity College Dublin.
“You can’t have physical controls along a border without customs men and women. If they come under threat, the only way you can protect them is to provide security forces. That’s very alarmist, but I don’t think there’s any harm in being alarmist at present.”
If there has to be Border, then one option is “to move it to the middle of the Irish Sea”, as Halligan puts it.
That would mean Border checks between Britain and Ireland but not between North and South – a scenario that would dismay unionists but help avoid the political and security headaches of a return to Border posts.
Another idea currently in circulation is "joint authority", which the SDLP endorsed in recent weeks as an alternative to direct rule from London in the event that the Stormont executive could not be re-established.
The proposal has its roots in a 1972 SDLP document, ‘Towards a New Ireland’, which set out how Dublin and London could share sovereignty under the international legal principle known as condominium.
Although used mostly for seas and lakes, condominium has occasionally been adopted by states to share dominion over territories; Anglo-Egyptian Sudan was an Egyptian-British condominium from 1899 until 1956, and the Brcko District of Bosnia Herzegovina is a condominium between the Federation of Bosnia and Herzegovina and the Republika Srpska.
Under the 1972 proposal, two commissioners, one each from Dublin and London, would jointly sign all legislation passed by a Belfast Assembly.
A new constitutional court comprising three judges – one appointed by each commissioner as well as the Chief Justice of Northern Ireland – would be created, while a new 50-seat senate, half its members drawn from the Dáil and half from the Northern Assembly, would work to bring into line the policies of both parts of the island on matters of common interest, such as electricity, tourism and regional planning.
Then taoiseach Garret FitzGerald broached a similar idea in the early 1980s, and in 1992 a leaked British government paper proposing a model of joint sovereignty provoked fury among unionists.
They remain implacably opposed, seeing it as a repudiation of a basic principle of peace process as constituted since the early 1990s.
In The Irish Times this week, columnist Newton Emerson called the idea "a fantasy".
Many close observers agree, but it’s a measure of how Brexit threatens to alter the island’s political landscape that such ideas are once again being raised.
In the early 1970s, John Hume situated the joint authority plan in the context of Irish and British accession to the European Economic Community (as it then was).
In January 1973, just two weeks after Ireland joined the European club, Hume predicted that the act of signing up for the European project was “going to create greater pressures for unity, particularly economic unity”.
As it turned out, its effect was more stabilising than disruptive. The peace process and the historic accommodation it produced between nationalists and unionists was made possible by the buttressing effect of Irish and British membership of the European club.
Now, more than 40 years on, it’s the sudden severing of that European connection that has left the island’s grand constitutional bargain at risk of unravelling.