They weren't cast for how similar they looked to the real life counterparts. I always thought Ray Liotta was the weakest element of the main characters; he just doesn't have the acting chops of De Niro and Pesci nor the charisma. I think someone like Sean Penn or even ( don't laugh) Matt Dillon or even Johnny Depp would've been better as Henry Hill.

Anyway, DeNiro is actually more Irish than Italian. He's only 25% Italian from his father, whose half Italian and half Irish. He actually is very fair skinned, so much so in fact that his nickname growing up was Bobby Milk.