Welcome to colonism. Have a look at africa, the borders there are a bit more arbitary.
But in ireland it is not only about the english living in north ireland, but more about the irish living there. It was more about catholic vs. protestant.
Not so easy to dissolve, but I hope the last blood happened long ago enougj for both sides to forget and move on, even if england messes up the deal amd the border becomes an issue again.