| I've heard that the prefix Mc- or Mac- in a person's name can be replaced by son of-. For example: McArthur can be understood as son of arthur. I think the intention was to say what clan you belonged to. So if you met Pidi from the clan of the Cloud, you could call him Pidi McCloud. The later use in family names is pretty natural from there.
ooh! just found a node mentioning that: Irish Gaelic family names
Update: gleeme says: a similiar prefix Fitz- (e.g. as in "Fitzpatrick") means bastard son of while Mc- means legitimate son of. | 
