Yes, GerMan, I think it is a more general phenomenon.
Take Germany and Austria, one big and one small country, close to each other geographically.
Germans feel closer to Austria than to, for instance, France though both border to Germany. Germans in their feelings are kind of inclusive (in sports: if none of us can win, we shout for the Austrian) and somewhat paternalistic. It just doesn't feel like going abroad when going to Austria. The feeling is: yes, we know in principle you have your own country but admit you are half Germans.

Austrians, on the other hand don't reciprocate these feelings of closeness. They want to demonstrate their independence by being explicitely anti-German. So in sports, seeing a German loose is nearly as good as seeing an Austrian win.

Do I exaggerate. Yes, of course, and for many individuals on both sides of the border it is different. But what you describe about Britain/Ireland reminds me in some aspects of Germany/Austria. I think the big/small country difference with an in parts common history can explain a lot.

Wolfgang