As Etymology for Mc and O' in names says, Mc is an abbreviation for Mac, and Mac is a Gaelic term meaning "son of". Hence the name McKenna means "son of Kenna", and since Kenna is a proper name, it is capitalized, even when combined with Mac.
(来源:
http:// english. stackexchange.com/questions/29450/why-is-the-letter-after-mc-in-names-capitalized)