O'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 and Kerry before the Anglo-Norman invasion. It is the third most numerous surname in Ireland. Due to emigration, it is also common in Australia, North America, Britain, and the rest of the world. According to traditional genealogy, the O’Sullivans were descended from the ancient Eóganacht Chaisil sept of Cenél Fíngin, the founder of the clan who was placed in the 9th century, eight generations removed from Fíngen mac Áedo Duib, king of Cashel or Munster from 601 to 618. 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. The last independent ruler of the clan was Donal Cam O'Sullivan Beare, who was defeated in the Nine Years' War of 1594–1603. Etymology and orthography consists of ó (Old Irish úa) "grandchild, descendant", and the masculine genitive case of Súileabhán, viz. Súileabhán's grandchild/descendant. The female form in Modern Irish is Ní Shúileabhán; "ní" is the shortened form of iníon uí, iníon "daughter", uí, the genitive of ó "grandchild, descendant". The etymology of the given name is uncertain. In his book titled The Surnames of Ireland, genealogist Edward MacLysaght states that “while there is no doubt that the basic word is súil (eye) there is a disagreement as to the meaning of the last part of the name.” It is interpreted as súildubhán ⇄ “little dark-eyed one” by Woulfe in Sloinnte Gaedheal is Gall, from súil ⇄ "eye," dubh ⇄ "dark/black," and combined with the diminutive suffix -án. Other suggested etymologies include "one-eyed" and "hawk-eyed." The original bearer of the name, one Suilebhan mac Maolura, is recorded in legendary Irish genealogy as belonging to the 8th generation after Fíngen mac Áedo Duib, and placed in the 9th century. MacLysaght lists Mac Criomhthain (MacCrohan) and Mac Giolla Chuda (MacGillycuddy) as notable branches of the Súileabhánaigh in County Kerry. O'Sullivan is the regular anglicization of the Irish name. Less common spelling variants of the name include: Sullavan, Sullivant, Sillivant, Silliphant, and Sillifant. Naming conventions Sullahan Some O'Sullivans in the midlands and south Ulster were originally (O) Sullahan (from Ó Súileacháin, probably from súileach, quick-eyed, according to MacLysaght). This surname has now almost entirely changed