Is "Mc" simply an abbreviation of "Mac" in a surname - or is there more to it than that?

As I understand it, they are both variants of the Gaelic word for "son of." Orthography was anything but standardised way back when. My own Irish anestors were surnamed Poland, variously spelled Polen, Polin etc. - even parish priests were not too well-endowed with literacy in 18th-century Ireland it would seem.