http://en.wikipedia.org/wiki/Kennedy_(surname)

Kennedy, alternately O'Kennedy and Kennedie, is a surname of Irish and Scottish origin that has also been used as a given name.

There have been several different etymologies given for the surname. One is that the name is an Anglicisation of the Gaelic Ó Ceannéidigh meaning "grandson of Ceannéidigh". Ceannéidigh is a given name derived from the Gaelic words ceann, meaning "head", and éidigh, meaning "ugly".[1] In some etymologies, the element ceann is given as "chief" or "leader".[2] Another possibility is that Kennedy is an Anglicisation of Ó Cinnéide, meaning "grandson of Cinnédidh" or "grandson of Cinnéidigh", with both of these personal names meaning "helmet headed.”[3] Ceanéidigh could be related to the old Gaelic name Cennétig, which is known from Cennétig Mac Lorcáin, the father of the Irish high king Brian mac Cennétig, who himself was also known as Brian Bóruma or Brian Boru. There are also an Irish Kennedy family and a Scottish Kennedy clan of Carrick in Ayrshire, which are unrelated to one another.