GardenGeek
There can't be a hard border between the Republic of Ireland and the rest of the EU because that would completely undermine the EU single market. Not gonna happen.
But there can't be a hard border between the Republic of Ireland and Northern Ireland because that would be a fundamental breach of the Good Friday Agreement, throwing people's lives in both countries into turmoil and putting the peace process at great risk. That is why Theresa May agreed last week - because there was no way the EU would agree to move on to trade talks otherwise - that there will have to be regulatory equivalence between Northern Ireland and the Republic of Ireland, both on the day we leave and going forward into the future.
This means that from a trade and regulation point of view, Northern Ireland must do whatever the Republic of Ireland does, which means it must do whatever the EU does.
But putting up a hard border between Northern Ireland and the rest of the UK would also be likely to stir up the troubles again because there is (just about) a majority of people in Northern Ireland who identify as British and not Irish. The DUP (who are currently propping up Theresa May's weak and wobbly government and without whom she does not have a working majority) have made it clear that there can be no solution which involves Northern Ireland being treated differently to the rest of the UK. If the DUP don't get what they want and withdraw their support from the Tories, the government will literally collapse.
So, to recap. The Republic of Ireland must do whatever the EU does because of EU law. Northern Ireland must do whatever the Republic of Ireland does because of the Good Friday Agreement (which the Government has said it can honour). The rest of the UK must do whatever Northern Ireland does, otherwise in the immediate short term the Government will collapse, and in the longer term a whole load of shit will kick off in Northern Ireland.
And so the only workable solution is that the UK must do whatever the EU does, without having any further say in what the EU does.
How's that sovereignty working out for us, eh?