The original Irish is Ó Suileabháin, deriving from súil (eye). The dispute over the meaning of the remainder of the name is understandable, since the two principal alternatives are "one-eyed" or "hawk-eyed". A further alternative, proposed by Diarmuid Ó Murchadha, gives their ancestor as Súildubán ("dark-eyed"), chief of a branch of the Munster Eoganachta tribal grouping, descended, along with such prominent families as the MacCarthys and O'Callaghans, from the mythical Eoghan, supposedly one of the original Gaelic invaders.
In historical times, their exact descent is more difficult to trace. According to some accounts, they were originally based in south Tipperary, around Knockgraffon, but by the beginning of the 13th century were firmly established in the areas which they are still associated, in the south and west of the modern counties Cork and Kerry. The move was almost certainly the result of encroachments by the O'Briens and the Norman invaders. By the end of the 14th century the family had split into at least seven different groupings. The most important of these were the Clann Gilla Mochuda of south Kerry (who changed their surname completely to McGillycuddy in the 16th century), the O'Sullivan MÓr, based on the shores of Kenmare Bay, and the O'Sullivan Beare, rulers of the area around Bantry and of the Beara peninsula in Co. Cork.
Donal O'Sullivan Beare (1560-1618) was one of the few Gaelic chiefs in Munster to support O'Neill at the battle of Kinsale. After the defeat he undertook, with 1000 followers, an epic trek 200 miles north to his allies the O'Rourkes of Leitrim. Only 35 survived to reach safety. He died in Spain in 1618, but the title survived and has been revived by the Spanish nobleman the Count de San Estaban de Caňongo.
Despite the defeats and dispossessions the numbers bearing the name have grown in their homelands; even today, four out of five families of the name still live in the two counties of Cork and Kerry, where O'Sullivan is the single most common surname.