Cool thread, great question.

The others have answered but I'll add that there are more 'Macs' in Scotland than in Ireland. In Ireland there are lots of O's ! Try looking in a phone book!

http://freepages.genealogy.rootsweb.ancestry.com/~kirkkilclansna/ScottishCountiesWithClansMap.png This may not be the best map of clans in Scotland. You can see that the names of most Scottish clans north of a line between Glasgow and Edinburgh begin with "Mac", though there are a few "Mc"s too. Interestingly, both MacDonald and McDonald are common in Scotland.

It comes from the Gaelic word 'Mac' which means 'son of.' It's common in Scottish and Irish surnames, yes. So McDonald originally meant 'son of Donald.' I'm sure you get the picture.

MoiraWendy, that just fired a random synapse that brought up a song fragment from some versions of "MacNamara's Band." It's a bit of patter that lists a series of stereotypic Irish names. Googling to find it, I learn that it may really be from a different song, "Dear Old Donegal." Meet ... Branigan, Flanagan, Milligan, Gilligan, Duffy, McGuffy, Malachy, Mahone Rafferty, Lafferty, Donnelly, Connelly, Dooley, O'Hooley, Muldoney, Malone! Madigan, Cadigan, Lanigan, Flanigan Fagin, O'Hagin, O'Hoolihan, Flynn Shanihan, Manihan, Fogarty, Hogarty, Kelly, O'Kelly, McGuinness, McGuinn.