Originally Posted by teksonline

When in doubt, hit a wiki

http://en.wikipedia.org/wiki/Mick

"Mick"
Many Irish surnames begin with "Mc" or "Mick", as in McDowell, or McShane, from which people from Ireland often get the nickname "Mick". (Mc usually means "son of". For instance the surname MacDonnell literally means son of Donnell.) Many Irish people are also named after the Irish leader Michael Collins, making Michael (Mick) a very common name.

It is sometimes used, usually with a lower-case "m", as an offensive slang term in British English and American English for an Irish person or a Roman Catholic. This is unrelated to the name Michael, however.

The term "mick" is also used as slang in the phrase taking the mickey or take the mick meaning to tease