The outcome of the 2016 British referendum on EU membership will have significant and lasting consequences. For the United Kingdom and its relations with European neighbours, for the constitutional fabric of the British State and for the EU at a time of uncertainty over the future of the European project. The consequences of this decision will have no greater impact however than on the still-fragile peace process known as the ‘Good Friday’ or Belfast Agreement, negotiated in 1998 by parties representing Northern Ireland’s principal cultural communities and the governments of the United Kingdom and the Republic of Ireland. This historic event brought to an end decades of political violence and centuries of sectarian bitterness, or so it was thought at the time. Brexit has thrown into doubt the future of that peace process.