I'm not a fan of the mix and match thing myself... MAYBE if it was a "daywear" jacket, but not on a Brian Boru (for those that are reading this and don't know, a Brian Boru is like a Prince Charlie, but Irish and slightly different in cut... very formal)

I think that if I couldn't get the buttons that are already on the jacket (get matching ones), I'd buy enough to re-do the whole jacket so they all matched. I just think it's a more "polished" look.