Hebrew, in case it isn’t. Verified by genetic testing of the x chromosome of men named Cohen, or its linguistic variants in most of the world. The priests of Israel were all the descendants of Aaron. Not counting the usual incidence of infidelity, they include a lot of guys named Cohen.
I’m pretty sure this is a myth. I believe Mc is just a popular ( and modern )shortening of Mac, not, as far as I know, a genuine linguistic variation between Irish and Scottish Gaelic. Every historical Irish name I’ve ever seen is written as Mac-, whether it be MacCarthy, MacLochlainn, MacDermot, MacMurrough or MacDuinnshleibhe.
As Tamerlane says, that’s a myth. It seems to have originated in North America, and is prevalent enough that both Scottish and Irish genealogy sites feel it necessary to address the issue.
I’m UK born-and-bred, with a “Mc” prefix courtesy of my Scots father, yet had never heard of the supposed “McIrish/MacScottish” correlation until I came to the US; one fellow was quite insistent that my father must have been from Ireland due to my “Mc” prefix, until I pointed out that my ancestors within recoded history were illiterate crofters (known rather more for their cattle-rustling and murder-for-hire proclivities than for their orthography), and that family gravestones in our local Highland kirkyard mixed “Mc” and “Mac” apparently at whim.