Ethnic groups don't actually have collective interests. Individuals have specific examples of universal interests. Sometimes individuals are lead to believe that they have collective interests, but that's usually because they're being made to do something against all of their individual interests. Let me offer a few examples.
Civil rights is a specific example of a universal interest: equality before the law. The rise of the Nazi party is an example of people forsaking their own interests for a facade of collective interest that covered over the personal interest of a few leaders - Nazi Germany was extraordinarily corrupt, and of course ruined the lives of and killed most of the people who it claimed to exist for the interest of.
It is interesting that you bring up "Catholic interests," because the Church is naturally opposed to concepts like "Irish interests." The Church doesn't want its members to divide themselves along ethnic or other lines because that would detract from their Catholicism. It is no accident that the Nazis - the most famous example of an "ethnic interest group" - had to destroy or subsume every other kind of organization to exist.
