The idea is to not to have to build a wall between the Irelands. A flip side implication would be that the EU is less interested in Scotland leaving the UK and joining the EU.

However, Northern Ireland would have trouble getting a referendum to quit the UK. By the power-sharing agreement, the Protestants would never leave and become the minority in a Catholic country, so no referendum would ever be held. And if power-sharing fails, the NI government reverts to direct rule from London, which would never let a referendum be held either.