Now I'm by no means an expert on the EU (far from it) but from what I've gathered from the KotOR games and the Clone Wars series is that there's a difference between being a Mandalorian and a "Mandalorian", at least by the time of the films. On the one hand you have the Mandalorians who are a quasi-nomadic warrior culture, big on personal honour, glory in battle and mercenary work. IIRC these aren't any particular species as there's a cultural tradition of adopting just about anyone they deem worth (i.e. can throw a punch and/or shoot straight.) On the other hand there's the people who actually live on Mandalore who--according to TCW--appear to be trying to shake their millennia old reputation and heritage as a warrior culture in order to join the Alderanian pacifists' club. Again, I'm not clear on the details, but I think between the Clone Wars and the rise of the Empire both these groups were put through the grinder so by the post-RotJ period they'd been--for better or worse--reunified.