Wikipedia
O'Sullivan , also known as simply Sullivan, is an Irish Gaelic clan based most prominently in what is today County Cork and County Kerry. The surname is associated with the southwestern part of Ireland and was originally found in County Tipperary before the Anglo-Norman invasion. It is the third most numerous surname in Ireland. Due to emigration is also common in Australia, North America, Britain and the rest of the world.
The O'Sullivans are the medieval and modern continuation of the ancient Eóganacht Chaisil sept of Cenél Fíngin, being descendants of Fíngen mac Áedo Duib, king of Cashel or Munster from 601 to 618. They are thus understood to be of royal extraction. Fedelmid mac Crimthainn (died 847), the celebrated King of Munster and nearly High King of Ireland, was the last king of the Cenél Fíngin/O'Sullivan line. Later they became the chief princes underneath their close kinsmen the MacCarthy dynasty in the small but powerful Kingdom of Desmond, successor of Cashel/Munster.