The other 26 countries can’t let the Republic of Ireland suffer with the wall built in NI.
The best option is to cut NI free and let the island of great britain with the Isle of Man, Orkneys, Jersey and Guernsey and Sark go out of the EU and the custom’s union. A big question mark over Gibraltar though
The Brits dont want a hard border in Northern Ireland. Neither do the Irish.
If one is to be built, the EU will have to do it. There is obviously no need to cut off a part of the UK from the UK.