My Irish surname book says that it is fairly rare, found mostly in Eastern Ulster. It's from Scots Gaelic Mac Fhionnghain (fair-born). It also says it's a family connected with Iona.
Just as a note, many Irish Mc and Mac names that are followed by a hard G, K or C often derive from words that look and sound more like the word after the hard G, K or C sound. Also some words that start with a Hard G, K or C were originally Mac names that dropped the Ma-. It's similar to what happens in Welsh when the Mac becomes Map, giving us names like MapRichard which ultimately became Prichard and MapHugh which eventually became Pugh.