Whoops, posted before finishing - only thing I'm a bit dodgy about is modern Scottish Gaelic using 'O''. It easily may have done in the past. But my wonderful informant's first language is the Gaelic, and she was pretty clear about 'Mac/Mc' being Scottish 'O'' being Irish. By which I mean: she easily could have been mistaken about whether the Irish use 'Mac/Mc' or not. I'm not so convinced she could be mistaken about whether the Scots use 'O''.

Could be regional. *shrugs* But I didn't think Hebridean Gaelic was particularly differentiated from mainland Gaelic.