Customs, Union

Oh, well, Northern Ireland did vote Remain. In that it is a separate legal jurisdiction, then there is is already a border between it and Great Britain, as there is a border between Scotland and England, and as there is a border between England and Wales. Northern Ireland barely trades with the Great Britain in the latter's vastly larger terms. Northern Ireland barely trades at all.

But even all of that, in itself, is beside the point. If the Parliament of the United Kingdom legislated that Northern Ireland should be in the European Single Market and Customs Union, while Great Britain should not be, then, as the sovereign will of the Crown in  Parliament of the United Kingdom, such would then be one of the terms of the Union. And that would be that.