Fingin reigned as the 14th Christian King of Munster. From Fingin descended the O'Suilebhain fmaily anglicised O'Sullivan and Sullivan. The root of this surname is the Irish Suil "the eyes" which is derived from the ancient Irish "sul" from the Lattin sol because the eye is the light of the body. The old Irish called Sunday Dia Suil before the Christians callid Dia Domhnaigh [ The Lord's Day].
According to O'Heerin's Topography, the O'Sullivans, before they settled in Kerry, were princes of Eoghanacht, Mor, Cnoc-Graffan, a territory in the Barony of Middlethird, county Tipperary, which is said to have embraced the districts of Clonmel, Cahir, Clogheen, Carrick-on-suir, and Cashel of the kings, in the fifth and sixth centuries.