the existence of a single white mascot doesn't invalidate the notion that numerous native american mascots are racist or offensive.
generally, i think it's up to the impacted party to determine whether or not they are offended by something. i've never heard anyone complain about the notre dame name/mascot and know of no effort or movement to change it. in fact, my irish friends seem to think it's cool. regardless, just because irish people are not offended by the notre dame mascot, it doesn't mean native american groups can't be offended by the redskins. it's a false equivalency.