They said on the commentary Moriarty is an Irish name, and they wanted to have an Irish accent for once.
I saw that. Funnily enough, I recently read Kim Newman's excellent Holmes Pastiche, A Volume in Vermillion, which is basically The Adventures of Prof Moriarty, told from the point of view of Sebastian Moran, Moriarty's henchman; think Flashman meets Holmes. In it, Moran acknowledges that both Moran and Moriarty are Irish names, but is at pains to stress that neither of them are Irish - which he does in no uncertain, somewhat politically incorrect terms! lol