CROSSING the border between Northern Ireland and his home in the south always left the young Gabriel D’Arcy sweating. But that is because he was invariably overdressed. In the 1960s the D’Arcy family made regular sorties to the north to buy cheap clothes; on the way home the children would layer on their purchases and sit bulging in the back seats as grinning Irish customs officials pretended to check the car for contraband. Today, as the CEO of LacPatrick, an Irish dairy firm with complex supply chains that zigzag across the border, Mr D’Arcy worries that Brexit will revive dangers that people on both sides thought had been consigned to the past.

The adamantine certainties of the Brexiteers are an ill fit for the ambiguities of Northern Ireland. Talk to any Irish official and one assertion soon surfaces: Her Majesty’s Government has not thought this through. After nearly 18 months of rumination, it is clear that there is no way to reconcile Britain’s aims of yanking the province out of the EU’s single market and customs union, and maintaining its seamless border with the republic, across whose 300-plus crossing points 110m trips are made annually.

The issue is pressing. Britain planned to delay tackling the border issue until the start of trade talks with the EU it hopes will get the go-ahead at a summit next month. Instead, Leo Varadkar, the Irish taoiseach (prime minister), and Simon Coveney, his spiky foreign minister, have threatened to veto that if Britain fails to offer “tailor-made solutions” for avoiding a hard border. Suddenly the Irish issue has become the biggest obstacle in the Brexit talks. Making matters more ticklish, Theresa May’s government in London is propped up by Northern Irish unionists hostile to any manoeuvre that weakens their link to the mainland. Northern Ireland itself is in crisis, without a government since January.

Ireland’s membership of the EU, which it joined alongside Britain in 1973, helped diversify the economy away from its larger neighbour, especially after the single market was established in 1993. In time it also provided a basis for improved Anglo-Irish relations. Brexit not only threatens Irish prosperity. It also places two countries that had been enjoying a golden age of good-neighbourliness on opposing sides of the negotiating table and in the cross-hairs of each other’s tabloids (Mr Varadkar has been instructed by the Sun, a British red-top, to “shut his gob...and grow up”). But Ireland’s EU membership at least leaves it better placed, diplomatically and economically, to weather the storm.

Northern Ireland, every border region of which voted to stay in the EU, labours under rawer concerns. If the 1998 Good Friday Agreement, which helped end the decades-long Troubles, failed to lessen the sectarianism that had fuelled the violence, it diluted touchy questions of identity. Under its provisions the Northern Irish may be citizens of Britain or Ireland, or both. The province enjoys a status distinct from that of the other British nations, guaranteed by both Britain and Ireland. North-south institutions govern matters of joint concern, from food safety to waterways. The agreement’s “consent principle” gives the north’s republicans a path to reunion with Ireland, should they win majority support for it. And without checkpoints, border regions have found that commerce, in Mr Coveney’s words, can be “a great healer”. Today the councils of Derry-Strabane (in the north) and Donegal (across the border) exploit the common regulatory space that EU membership provides to market themselves to investors as the single economic unit of “Ireland North-West”.

Brexit undermines all this, with particularly grave economic consequences for the still-struggling north. It is “quite remarkable”, says John Bruton, who as taoiseach in the mid-1990s helped create the conditions for the peace settlement, that French, German and Italian leaders take the Good Friday Agreement more seriously than their British counterparts. A return to full-scale violence is not in store. But Jim Roddy, a community leader in the Northern Irish border city of Londonderry, says low-level paramilitary and gangland activity “has increased significantly” over the past year. He fears Brexit could make it worse.

An unpalatable choice

If Brexit is a tragedy for Ireland, it has at least offered its officials a chance to shine. No European government has prepared better. Irish diplomats quickly convinced their European colleagues to treat the border as a priority in the Brexit talks, and have held the line since. Now, as Mr Varadkar seeks to exploit his moment of leverage, his challenge is to avoid overplaying his hand—by shortening the odds of a no-deal Brexit that would be ruinous for Ireland, or by testing the patience of EU countries that do not see the border issue as existential.

Tired of the platitudes they hear from British politicians, the Irish are pushing their own ideas. They want written assurances from Mrs May that would in some fashion lock the north into the EU’s regulatory and customs system, reducing the need for border checks. Irish and EU officials are studying existing areas of all-island co-operation, such as electricity, as possible models. Some look to Hong Kong or Macau, which have independent trade policies from China. The EU is on board: this week Michel Barnier, its Brexit negotiator, said Northern Ireland required a “specific solution”, and that Britain must offer proposals soon.

To unionist-minded Britons, this is the sort of foreign meddling that inspired them to leave in the first place. The concern is reasonable: the Irish deny any designs on the north, but any scheme that binds it to the republic will inevitably mean drift from the mainland. Yet the dilemma highlights the point. The British helped lay to rest the ghosts of the past by removing from the people of Northern Ireland the need to make a clear binary choice between Britain and Ireland. Their vote to leave the EU rips that constructive ambiguity apart. Some questions are best left unanswered. But Brexit renders that impossible.