McDermott is an Irish surname of Gaelic origin and is a patronymic name derived from the Gaelic Mac Diarmada, meaning “son of Diarmuid.” The personal name Diarmuid is composed of the elements di, meaning “without” or “divine,” and armait, meaning “envy,” or alternately ar, meaning “without envy,” thereby rendering the meaning “free man.”
The name is most closely associated with the province of Connacht, particularly County Roscommon, and with the province of Ulster in the northern part of Ireland. Two prominent branches of the family formed in the medieval period: one settled in Coolavin, County Sligo, the other in Kilronan, County Roscommon. The lands they held were historically referred to as Mac Dermot’s County and were part of the traditional territory of the clan.
The clan’s traditional ancestor is Diarmuid, the High King of Ireland together with Blathmac in the year 657 A.D., and the name also references Dermot, the King of Connacht in the twelfth century. The chief of the family is recognised by the Irish Genealogical Office. He is entitled to be called “The Mac Dermot” and bears the title Prince of Coolavin.
In the annalistic tradition the Annals of Lough Ce record the deeds of Brian Mac Dermot, a 14th‑century chieftain, and his work was an inspiration to later chroniclers. A later member of the family, Martin Mac Dermott (1823‑1905), was noted as a Young Irisher and a poet of the nationalist period of The Nation. The earliest documented spelling of the name is found in an entry concerning Brian Mac Dermot, who died at Moylurg in 1592.
Variations of the surname include MacDermot, MacDermott, McDermot, McDermitt, McDermod, McDermaid, McDermond, McDermid, McDermotroe, as well as shorter forms such as Dermott, Dermot, Dermody and Dermond. The spelling has often been altered through anglicisation and migration, but all variants descend from the same Gaelic root.
Historical migration, particularly during the Great Famine of the mid‑nineteenth century, spread bearers of the name throughout the Irish diaspora. Significant concentrations of people with the surname now exist in the United Kingdom, the United States, Canada and Australia. Despite this dispersion, the name remains common in Ireland, especially in the western regions of its original provenance.