No marriage lasts forever. Brexit is the kick in the pants Scottish independents and Irish republicans have been waiting for.
Since only England voted for brexit it's quite conceivable that we'll eventually end up with united Ireland and Scotland both joining the EU with England and Wales left on their lonesome.
One major hurdle for that is that in order to join the EU, a new member will need unanimous support from all current member states. I think Spain has already stated they are opposed to any breakaway state joining the EU. They essentially want to prevent a possible independent Catalonia from being able to leave Spain and join the EU. I think a couple of other member states are in a similar position.