What would a UK vote to leave the European Union mean for Ireland? Leave supporters rarely bother to ask this question. But they should, because the answers matter. The Irish republic is our closest neighbour, while Northern Ireland, part of the UK, is still haunted by 30 years of the Troubles. More than half a million Irish people in the UK, and the whole adult population of Northern Ireland, can vote on 23 June. On both sides of the border, majorities of Irish people want Britain to remain in the EU. This week, John Major and Tony Blair, who both helped to bring peace to the north, warned that Brexit could put this work at risk. If nothing else, the leave camp needs to explain why it thinks such views should be ignored.

There are four main reasons why voters in Britain should think about Ireland and not flirt with a leave vote. The first is the benefit to Northern Ireland of EU membership. Ireland’s and the UK’s shared place in the EU has been a confidence building context and a large financial support to the peace process. It provides a cooperation framework between the two sides in the north. The EU has provided cash, too – more than £2bn to Northern Ireland in the six years to 2020. That would end with Brexit.

Political uncertainties in the north after Brexit may be less tangible, but they are no less worrying. All the pieces of the peace process jigsaw would be thrown in the air, said Sir John Major this week. Who can say with certainty that he is wrong? Sinn Féin has said a new vote in the north on unification would be necessary. That would threaten the power-sharing arrangements. The danger of violence might increase – uncontainably if such a vote went the republicans’ way. The English leavers who talk so airily of getting their country back might find they were getting more back than they bargained for.

Mr Blair stressed a third issue. With a Brexit the two parts of Ireland would be on different sides of an EU land border. Given the leave campaign’s concern about borders, the current ease of crossing could hardly survive. Border controls and customs checks could follow, bad for people and trade. The travel area between the republic and the UK might have to be rethought. Ireland might have to join Schengen. Either way, ports and airports would have to be policed in new ways on both sides of the Irish sea. A post-Brexit government would not be likely to give priority to Irish interests, north or south.

The final effect would be on the British-Irish relationship generally. One of the great achievements in these islands over the past half century has been the organic evolution of far more mutually respectful relationships between the British and the Irish after the centuries of hurt. That has been embodied in this year’s absence of recrimination in the Easter Rising centenary and by the shared embrace of next month’s Somme centenary, too. Even a Brexit would be unlikely to halt that process in its tracks. But it would not assist it either. A Brexit drawbridge drawn up against the world would be drawn up against Ireland as well. It would be another needless closure of the English mind in particular.

Are all these things what leavers want? If so, they are even more irresponsible than they already seem. The Irish dimension will not be the key issue on 23 June for most voters. But it should not be ignored either. Here, as on so many other grounds, a vote for remain is the far better course for Britain.