I thought that border did close briefly? I understand it's a touchy issue, but here we had some rather soft closures between states as well, borders don't have to be binary opened or closed. In some instances people could work permits to cross the border, for others we made virtual borders so people that live near the border could cross but no one else could. At various times we've thrown up some new borders, people couldn't travel from Melbourne to regional Victoria for instance.

Would options like that be viable with NI?

Remember this is somewhere that border checkpoints have in living memory been the target of violent attacks, where the idea of "partition" is at the core of the disagreements.