Origin: Irish. The name McGillycuddy in Ireland is taken from the Gaelic Sept Mac Giolla Chuda who were a branch of the Kerry based O'Sullivans. The chief of the Sept was MacGillycuddy of the Reeks, who gave his name to the area of the Sept homeland of Kerry. The surname comes from the Irish MacGiolla 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 to 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.

Spelling variations include: Gillycuddy, Gillicuddy, Gillecuddy, Gillacuddy, McGillycuddy, McGillicuddy, McGillecuddy, McGillacuddy, McGullucuddy, MacGillicudy, McGillicudy, McGilly, MacGilly and many more.

Motto Translated: "Sursum Corda", Hearts upwards.