Etymology of Kennedy
There have been several different etymologies given for the surname. One is that the name is an Anglicisation of the Gaelic Ó Ceannéidigh meaning "descendant of Ceannéidigh". The Gaelic personal name Ceannéidigh is derived from ceann meaning "head" + éidigh meaning "ugly". In some etymologies the element ceann is sometimes given as "chief" or "leader". Another possibility is that Kennedy is an Anglicisation of Ó Cinnéide, meaning "descendant of Cinnédidh" or "descendant of Cinnéidigh", with both of these personal names meaning "helmet headed".
(wikipedia on Kennedy (surname)