> Republic of Ireland joining the United Kingdom

Sure, but this is somewhere in probability behind "UK destroyed by asteroid impact".

> Scotland making a bid to join the EU

There are persistent rumours that people on the UK side are trying to avoid having any kind of double-no-border situation that would work for NI be also applicable for Scotland. Because during the referendum all sorts of claims about the need for a hard border were made, often by the same people who claim there doesn't now need to be a hard border in Ireland.