I was not able to find a site to give me a lot of historical information on the Kennedy surname origin. I will post what I have found and suggest that anyone with more information forward it to me.

Gaelic: ó Cinnéide , Gaelic Meaning: helmet headed

The Irish surname Kennedy is derived from the O'Cinneide Sept that was based in County Clare in the West of the Country. Branches of the Sept migrated to Counties Antrim and Tipperary. Today the name is widespread throughout the Irealdn and in fact is among the 20 most frequently found.