The EU has to enforce its 4 freedoms
If it makes an exception in the case of the UK, then WTO rules and MFN clauses in its 40 FTAs mean that it would have to make an exception for all the other countries too
i.e. the EU would basically have to demolish its Single Market, on which its prosperity depends
So, yes it could theoretically choose not to enforce the rules, but only if it is prepared to destroy the prosperity of its own members,
to save the UK fro the consequences of its own decision
The Republic of Ireland and indeed the whole EU could theoretically choose not to enforce import controls on UK goods at its external borders,
but then again it would also have to abolish import controls for all the other countries
Unless the UK stops exports to the Channel ports, then they will soon be logjammed on the EU side, as they struggle to carry out all the required checks.
WTO rules and MFN don't prevent countries from having open goods borders; they just require them to be open for all countries on equal terms.