How do you define ethnicity and race? Apart from the ambiguous areas I do not think we all use the same definitions.

> In the US it is very common for someone to identify as half black and half white, or one quarter Irish and three quarters Mexican, and so on.

That is one of the things I find hardest to understand about American definitions. I would just think of someone like that as American (unless they are significantly culturally not American in some way). I just describe myself as mixed, or as from one of the two countries I am a citizen of (I have lived in both for significant chunks of my life and am influenced by both cultures but have little ancestry from either).