My own ethnic background is almost all Scottish and Irish, and I do not suffer from any "curse," nor do the other guys in my family. I have seen plenty of other guys who are either exclusivley or primariliy of Celtic background, and I have seen no pattern to exist one way or the other.
I beleive that all the talk about one's ethnic group having some baring on penis size is all myth, as is most other commonly expressed correlations (size of nose and feet, distance between index figer and thumb, etc.) You never know how well-hung a man is until he drops is pants.
The only pattern I personally have observed is that most of the east Asian men I have seen are smaller than other racial groups. Even then, I am sure there are men from east Asia for whom this is not true.