Yesterday I heard an interview with a dude named Guy P Harrison, who wrote the book Race and Reality: What Everyone Should Know About Our Biological Diversity. He said that when Irish immigrants first came to America, they weren't designated as white. They weren't good enough to be white. So that explains that.
The interview was pretty lengthy and it really gave me a lot to think about. I'd like to actually read the book before I talk much about it, but he says there's no such thing as race, scientifically speaking, and the differences we perceive between the races are due more to society than biology.