A decade ago, the Democratic Unionist party (DUP) viewed Brexit as a potential gamechanger in the struggle to resist future pressure for a united Ireland. As the author and journalist Fintan O’Toole wrote in an essay published last year, the DUP hoped that the 2016 referendum would deliver “a double exit. Brexit would not simply mean the UK’s departure from the EU. It would also be an act of separation from the Republic. It would reverse the slow drift towards integration.” That proved to be, to put it mildly, a deeply flawed judgment.

The direction of travel since 2016 has instead been the opposite. Rather than leading to a hard border between Northern Ireland and Ireland, which would have disastrously undermined the Good Friday agreement, the UK’s divorce agreement from Brussels eventually placed one in the Irish Sea. The logic of further economic integration........

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