The Irish surname Brennan is an anglicized form of two distinct Gaelic Irish surnames:-O'Braonain and MacBranan. The gaelic prefix "O" indicated "male descendant of", plus "braon", a "tear" or "drop", possibly meaning "sorrow" in this case. "Mac" means "son of", plus "bran", a raven, referring to one with black hair. The chief O' Braonain (Brennan) sept belonged to Leinster where they held considerable estates in Co. Kilkenny prior to the 17th Century, after which several clan members became "highwaymen". The MacBrennans belonged to Co. Roscommon and their chiefs held sway here from 1159 - 1488. Today there are approximately 48,000 people living in the United States with the last name Brennan.