Does EU now end at our Northern border?
At 7am yesterday, as Britain's going-going-gone result became apparent, a text message pinged from my 19-year-old niece in England. "Do you know what happened to Grandma's birth certificate after she died? I'd like to apply for an Irish passport because I really don't want to class myself as British right now. I'd much rather be Irish and European than British and not European. I believe in the EU and want access to it."
A university student with an Irish mother and English father, she had just woken up to Brexit. Back and forth travelled our texts. Some 75pc of her age group (18- to 24-year-olds) voted to stay, and she and the majority of her friends were "devastated" about leaving.
"We think reform from within would have been better," she said. "It seems unfair that our future has been decided by the over-55s - in some cases by elderly people who won't have to live with the consequences.
"But there are a lot of people celebrating right now who believe this is about self-rule. Hopefully we won't have destroyed the EU in the process. The general attitude has turned to discussing who will be the next prime minister and broker of deals."
The European project must surely be dented by Britain's decision, not least because of the domino effect from other countries gung ho to follow its lead. But the law of unintended consequences may lead to the breakup of another union - the United Kingdom.
The referendum which lost David Cameron his position - and will strip jobs from other people if a downturn follows - acted as a Pied Piper call to a sense of British identity. Except doubt is cast on that identity. We're not really European, says the result. But not all of us are British, either. English and Welsh voters are out of step with Scottish and Northern Irish - a cloud juts over the union.
Clearly, Scotland believes its best interests lie with Europe rather than Britain, as does Northern Ireland. Sinn Féin is champing at the bit to move this along to the next stage with a plebiscite on Irish unity. If such a referendum should be held in the North, it would trigger one in the Republic under the Good Friday Agreement's terms.
However, Sinn Féin would need to be confident about the upshot before pressing for such a ballot - referenda can deliver unexpected outcomes. Ask David Cameron.
"This does not mean that the UK will be any less united," Downing Street hopeful Boris Johnson said yesterday, in words which may bite back sooner than he thinks.
A few hours after my early morning hunt for our Limerick-born mother's birth certificate, the phone rang again. It was a colleague from Edinburgh, speculating that a Scottish referendum on independence could be run within two years. Scottish nationalists would press for one before the UK exit, to avoid Scotland needing to negotiate its way back into the EU. Equally, London could refuse Scotland another vote - it has that power. A dismayed JK Rowling, the 'Harry Potter' author who campaigned for Scotland to stay in the UK two years ago, tweeted: "I don't think I've ever wanted magic more." But the genie is out of the bottle, the wishes are all used up.
This has been a difficult referendum to watch from the Irish Republic vantage point because it affects us so closely - yet we were relegated to the sidelines.
The logic now for border controls seems inescapable - for customs posts, gardaí and security checks. For the re-emergence of that virtually invisible Border which we thought was consigned to history.
The North voted by 91,000 ballots to remain, with cross-community support, including from farmers reliant on EU subsidies now under threat. Both the North and the closely interconnected Border region will face economic challenges under this new arrangement. Obstacles to trade will be minimised, no doubt - the goodwill is there. But hurdles are inevitable.
Unemployment will be an issue for the North if foreign direct investment dries up - some 40,000 new jobs were created in recent years. Trade between North and South and Ireland and Britain will be impacted if tariffs are introduced, and sterling's collapse will reduce the important tourist trade from Britain to the Republic.
Yesterday's currency fall sent a shudder through the Border region, where people are particularly exposed to fluctuations, with some working in the North and living in the South and vice versa.
So it is that DUP First Minister Arlene Foster, who heads a party which advocated leaving the EU, finds herself out of kilter with the majority of those she leads. Inevitably, the poorest will be hardest hit. As for the 300-mile land border between North and South - is this now where the EU ends? The thrust of the 'Leave' campaign hinged on border controls, so Britain will be obliged to police its demarcation lines. It will have to decide whether to set its border around the island of Britain, or between Northern Ireland and the Republic. Unionists will be vexed by the former, while Nationalists will regret the latter.
Is there any bright side? Arguably, Brexit could be interpreted as Britain taking a leap of faith. Other Euro states, or groups within them, have chafed at the remote and authoritarian European institutions, and Brexit is an inspiration to them. As the EU has advanced towards ever-increasing unification - a superstate - calls for reform have been ignored. Some people have become alienated: the British were first to act on it.
But Britain is a loss to Europe, and to Ireland. Despite our fractured history, we learned to be neighbourly and were natural allies within the EU. We must remain natural allies out of it. Meanwhile, the current vacuum is marked by uncertainty which serves no one's interests. The sooner Brexit is thrashed out the better - and the EU has made it clear (somewhat to Britain's surprise) that exit should proceed at top gear.
It's not the only new relationship under negotiation. Ireland will have to mediate revised terms and conditions with Britain - a process which will gobble through time, goodwill and resources. Fasten your seatbelts, we're in for a bumpy ride.