I always figured the McClernan side of my family was straight-up pure Irish. The first time this idea was shaken was when I spent a week in Ireland in September 2000 (15 years ago already!) and had a peek at the Dublin phone book in the hotel - back when phone books were all the rage - and could not find a single McClernan mentioned therein.
It turns out my heritage on that side, at least, is more Scotch-Irish. According to the Ancestry.com web site there were a bunch of McClernans in Scotland according to the 1840s Scottish census.
There are also McClernans noted in Northern Ireland, that hotbed of Protesetantism. Also listed among the "Ulster Scots" here.
Now the Scotch-Irish were known as Presbyterians, but my family is straight up Catholic as far as I know. Of course there were Roman Catholics in Northern Ireland, which is one of the reasons for "The Troubles." Although if Wikipedia is to be believed, things have been pretty quite for the past 13 years or so.
Ultimately I don't actually care about where my family comes from. If you go back far enough we are all out of Africa anyway.