'Canonical' BDS spelling is MacManus, as enshrined by the script, the imdb.com listings and the credits on at least two of the four DVD versions. MacManus and McManus actually show up in both Scotland and Ireland--it's a matter of which one's more prevalent in a country. But most of us serious BDS people figure Troy Duffy (the director) was just being an idiot and never corrected it.

why, yes, i did do serious research on this topic. i even have a post. and i take this way too seriously:P