IN THE Brexit referendum of 2016, 56% of people in Northern Ireland voted to remain in the European Union. Nonetheless, the region will leave along with the rest of the United Kingdom on March 29th next year. The Republic of Ireland, of course, will continue to be a member of the club. This means that for the first time the EU will have an external border on the island of Ireland. What will be the political and economic consequences?

During the sectarian “Troubles” of 1968-98, the Irish border was for much of the time heavily fortified. People and goods crossing the 500km frontier were stopped for customs and identity checks, as British army helicopters whirred overhead. These days, in contrast, one can drive across the border without noticing it. Security measures were phased out following the Good Friday peace agreement of 1998, and the EU’s single market and customs union have banished the need for inspections of imports and exports between the two countries.

If the United Kingdom leaves the single market and customs union, as Theresa May’s government intends, those inspections will return in some form. But how? Ireland says it would not accept customs checks or infrastructure at the border, and Mrs May promises there will be none. She rules out the idea of Northern Ireland remaining in a customs union, aligned with single-market regulations, as this would have the effect of creating a customs border within the UK. The whole country could stay in the single market and customs union. But she rejects that as a watered-down sort of Brexit. The EU, for its part, dismisses Mrs May’s suggestion that technology could allow invisible checks at the border or that the UK could collect duties in Northern Ireland on behalf of the EU.

Mrs May will surely have to soften one of her “red lines”. But doing so threatens to inflame nationalist passions of one sort or another. A hard land border would appal Irish nationalists, for whom the open border is a crucial part of the Good Friday deal. Customs checks between Northern Ireland and the rest of the UK would upset Northern Ireland’s unionists, who are sensitive to anything that separates them. Keeping the whole of the UK in a customs union, meanwhile, would be seen by English nationalists as a betrayal of Brexit, as it would limit the prospect of trade deals with new countries. Whichever concession Mrs May makes, expect fireworks.