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O'Sullivan is among the top three of Irish surnames coming only after Murphy and Kelly in frequency. O'Sullivan is the anglicised form of the Gaelic Ó Suileabháin whose exact derivation is in doubt. Certainly the root of the name is 'Súil' meaning eye. However the additional stem 'eabháin' is variously translated as 'hawk-eyed', 'one-eyed' or even 'black-eyed'. Griffith's Valuation, a comprehensive listing of those who rented land/property throughout Ireland in the 1850s, records a total of 7858 occupiers named 'Sullivan' (with only a handful using the older form of 'O'Sullivan'. Of these 3388 were found in County Cork with the majority of the remainder being in the neighbouring county Kerry and two to three hundred each in counties Limerick, Tipperary, Clare and Waterford. This high concentration in the counties of the south-east accurately indicates the origins of the family. Descended from Eoghan Mór, they were one of the most powerful families of the Munster Eoghanacht. The Anglo-Norman invasion of the twelfth century pushed the family further south into the areas where they multiplied and divided. The two most important branches were O'Sullivan Mór based at Dunkerron Castle on Kenmare Bay and O'Sullivan Beare whose territories included the baronies of Beare and Bantry. In contrast to Griffith's Valuation, most who now bear the name in Ireland have reverted to the older form of O'Sullivan rather than Sullivan Various members of these families played pivotal roles in the later history of the Gaelic Irish in their struggle against the encroachment of English administration. Their eventual defeat led to their flight to Europe where they became prominent in the armies of the various empires. In more recent times, O'Sullivans have been to the fore in the history of the United States both militarily and politically. They have also played important roles in the development of Ireland in the nineteenth and twentieth centuries.