The UK needs an internal market, in the same way the European Union has the Single Market. All 4 Nations of the United Kingdom must be able to achieve friction-less trade with one another post-brexit.
It's equally important for international trade. If the UK had 4 sets of rules and regulations that third nations had to comply with. Future deals would be almost impossible to complete.
And again, we're hearing the horrors of implementing a Hard Border on the Island of Ireland. No one wants this. Therefor, no one needs to implement one.
The implementation of a 2-tier port system could be one of many solutions?
Designate a port in Northern Ireland, where all Irish Republic bound goods from the UK are exclusively sent for checks. Plus, all goods from Northern Ireland, bound for the south should also transit the designated port, far away from the actual border.
When the Republic of Ireland export to the UK, they should have the choice of transit via the designated port or ship directly to the main land.
Personally, I think the EU are parading themselves as the voice of reason. But, in actual reality they want the talks to fail, unless the UK signs up to being an EU satellite, in essence they won't be happy unless the UK follows their rule book.
If I were a betting man, I'd wager the collapse of talks and a No Deal by mid October at the latest.