I understand how borders work. My point is that the EU will not put a binder between Ireland and NI, if the UK doesn't, as it's too politically sensitive. They will not want to be the end of any possibility of unification, they will not want to be the cause of violence and they will not want to be seen to be dumping a member in the brick, just to keep their bureauzone intact.
The EU will absolutely put a border on NI if the alternative means that the integrity of the CU and SM is threatened.
Brexit is the UK's foly. Any fall out in NI or elsewhere and the blame will squarely be put on the UK.