I've always confused the two, and people seem to think they aren't too similar. But for some reason they always seemed like the same person to me even though Jeremy Irons seems to always play Jeremy Irons and Alan Rickman has a bit more range. However, it does seem like Snape and Galaxy Quest, Rickman is doing a kind of Jeremy Irons parody but maybe that's just my bias.

Anyway, I caught Die Hard 2 With a Vengeance on cable TV recently and realized the villain is Jeremy Irons playing Hans Gruber's brother from the first one. So maybe seeing these as a kid imprinted this connection on me even though Irons is blonde in it. Like they both just blended together as "Die Hard Villain" in my mind and I could never separate them.