>Anecdotal example: In Brazil, you don't see people claiming they are Irish when their families have lived in the US for generations.

That is actually the key to Americas success. The total separation of ethnicity and national identity. It allows an immigrant to keep their ethnic identity and merely add an American one.

It's also helps when a white 5th generation american thinks they are Irish and American. They'll be less likely to believe that a newly arrived Mexican isn't really American. They intuitively know you can be both.