Yes, Seamus is one of the main characters in Harry's year. He does have a tendency to be a buffoon, but so do Ron and Neville, so I don't think there's any ethnic bias. Again, it's hard to tell if anyone else is Irish, because ethnicity is not dwelt on at all.

There do not, by the way, appear to be any Americans at Hogwarts, or indeed among the prominent Wizards we meet. But we do find out that there are separate wizarding establishments in France, Germany and elsewhere, so we must assume that the American magical community has its own schools.