I'll have to hand this Gay Michel one point: It is indeed easy for agreements to get unagreed when one party is a small and weak country, no matter what international law or democratic legitimacy say. And in a very familiar one-two step the fact that you have to deal with material conditions and the real world leads to the inevitability of all his pet policies.
Now to the main point I'd argue that the only real way Irish unification realistically happens is if it is just too much of a pain for everyone to not do it. And with UK state capacity being in line for a rather precocious decline, mainly caused by the total victory of rapid reactionaries over even the most milktoast reformers that point might arrive in the not so far future. And obviously you shouldn't let yourself be scammed into accepting any nonsense conditions for solving one of the bigger UK and EU headaches.