Well, we needed the school to be located in the British isles for the British-style school system to make sense. But that was a relatively minor thing. What was not a minor thing, was that we wanted our school to make sense in-universe. With how many myths and magical legends go around the world - a lot of things from wizard world seeps in into the normie world. Distorted as it may be. A school with hundreds of students? Standing for hundreds of years? There has got to be at least some myths and legends about it. And it had to be something relatively major. Somebody got to have noticed. Some rumors got to have slipped past the charms. So, "Tir na nOg" was the perfect candidate. Children disappearing for some reason and returning back after several years? They must've been spirited away by fairies. Some normies vaguely remembering getting lost in a place full of magic called "Tir na nOg", etc. It just fits. And the fact that "Tir na nOg" translates to the "Land of Young/Youth", which is a very fitting description for a school, was just a cherry on top. So Ireland was just a perfect fit no matter how many times you slice it.