The question of what kind of border after Brexit will exist between Northern Ireland and the Republic will, I predict, become a very thorny one indeed as negotiations crawl into the autumn. Talk of ‘putting the border in the Irish Sea’ — somehow leaving the north inside the EU for customs and immigration purposes, but cut off from European funding — was a red herring that provoked DUP tantrums, but more significant was the weekend outburst from Taoiseach Leo Varadkar.