No, Windy; they mean the same. Mc is just an abbreviation of Mac. I think it became a ‘difference’ due to prejudices in some areas of the US, but it really has nothing to do with Scottish or Irish:
http://www.scottishhistory.com/articles/misc/macvsmc.html
Yeah. I know, but (at least in modern times) Mac has been associated with Scots, while Mc has been associated with Irish.
I've got both in my family. It gets confusing .. ;-)