Northern Ireland is part of the UK, and will remain so until a majority of people here decide otherwise.
He asks, "what principle is that exactly? That a majority in the six counties outweighs a majority in Ireland as a whole? What happened to the principle of national sovereignty".
Leaving aside the philosophical point that the author obviously feels it's absolutely self-evident what does and does not constitute a nation, a more direct answer. It's called the 'principle of consent' and it is accepted, North and South, by the vast vast majority of people in Ireland.