MacGillycuddy comes from the Irish Mac Giolla Mochuda, meaning "son of the devotee of (St) Mochuda". Its adoption was quite unusual. St. Mochuda, a pet form of Carthach, meaning "loving", was the seventh-century founder of the important monastic settlement of Lismore, in Co. Waterford. He was a native of Kerry, and when his fellow Kerryman Ailinn O'Sullivan became bishop of the diocese of Lismore in the mid-thirteenth century, he initiated the practice of the O'Sullivans paying particular devotion to this saint. As a result, the practice grew up among one of the leading families of the O'Sullivans of using Giolla Mochuda as a kind of title. The first to use Mac Giolla Mochuda was Conor, who is recorded as having slain Donal O'Sullivan Beare in 1563. His family, descendants of Donal Mor O'Sullivan, the common ancestor of O'Sullivan Mor and O'Sullivan Beare (see O'Sullivan), continued to be known as "MacGillycuddy O'Sullivan" or "MacGillycuddy alias O'Sullivan" well into the seventeenth century, when MacGillycuddy became established as a surname in its own right. Even at this point, less-well-off members of the family continued to be known as "O'Sullivan" for quite some time.
The family controlled a large territory in the Kerry baronies of Magunihy and Dunkerron; the name of the great mountains in Dunkerron, MacGillycuddys Reeks, preserves the record of their ownership. Members of the family retained large estates in the area down the the twentieth century. Unlike many other families of the old Gaelic aristocracy, their line of descent remains clear down to the present day; the current holder of the title "The MacGillycuddy of the Reeks", recognised as such by The Genealogical Office, is Richard Denis Wyer MacGillycuddy, now resident in France.
The original Irish is O Suileabhain, deriving from suil (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 O Murchadha, gives their ancestor as Suilduban ("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 13 th 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 in the 16th century changed their surname completely to McGillycuddy, the O'Sullivan Mor, 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 Ca?ongo.
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.