At last it’s becoming clear to the Brexiteers, and the right-wing British media, that the question of the Irish border will be the defining issue of the Brexit negotiations. Theresa May has long promised that the UK will be withdrawing from the single market and the customs union, while simultaneously promising no border on the island of Ireland. The simple fact is that she cannot keep both of these promises - something that the Republic of Ireland, the European Union, and large swathes of Remainers have long pointed out.