In the summer of 1585, a jury of Roscommon freeholders sat under the terms of the Composition of Connacht to inquire into the lands held by Tadhg mac Diarmata, the last man to rule as King of Moylurg. The inquiry was framed as routine administration — the crown was converting the old Gaelic lordships of Connacht into fixed rents and freehold tenures, family by family, barony by barony. For the Mac Diarmada, the outcome was anything but routine. The composition ended nearly six centuries of kingship in the barony of Boyle and reduced a ruling dynasty to landholders answering to English law. Sworn inquisitions like this one are the reason so much of the family's medieval and early modern history survives in written form at all — the same courts and clerks who dismantled the kingdom also, inadvertently, recorded it.
Quick answer: McDermot is a variant spelling of McDermott, the anglicized form of the Gaelic Mac Diarmada, meaning "son of Diarmaid." The Mac Diarmada were kings and later princes of Moylurg in County Roscommon, one of the most thoroughly documented Gaelic-Irish ruling families, with an unbroken line still recognized informally as "The MacDermot, Prince of Coolavin" today. Most historical sources render the surname with a double T; this article focuses on the single-T McDermot spelling and its place within that same family history.
Where Does the McDermot Name Come From?
McDermot descends from the Irish Mac Diarmada, literally "son of Diarmaid." Diarmaid was itself an old and widely used personal name in early medieval Ireland, later anglicized on its own as Dermot, Darby, or Jeremiah. As a surname, Mac Diarmada was not adopted loosely by many unrelated households the way some patronymic names were. According to the entry on the name in Edward MacLysaght's study of Irish surnames, the family "took the surname of Mac Diarmada" from a specific ancestor: Diarmaid, grandson of Maelruanaidh Mór and a man who died in 1159. Everyone who legitimately carries the name Mac Diarmada, in any of its later spellings, descends in principle from that one eleventh- and twelfth-century lineage in Connacht.
That lineage was not obscure. Maelruanaidh Mór mac Tadhg, the grandfather figure behind the surname, was a son of Tadhg mac Cathal, an eleventh-century King of Connacht. Around 956, in a negotiated settlement, Maelruanaidh gave up his own claim on the provincial kingship in exchange for the territory of Moylurg. His descendants, first known as Clan Mulrooney and later as Clan Mac Diarmada, kept close family ties to the O'Connor kings of Connacht for the rest of the medieval period. The surname therefore does not simply mean "son of a man named Dermot" in the generic sense — it points to a specific royal Connacht pedigree, which is part of why genealogists treat it as one of the best-documented Gaelic surnames in Ireland.
What Was the Kingdom of Moylurg?
Moylurg, Magh Luirg in Irish, was a Gaelic sub-kingdom of Connacht covering a substantial stretch of what is now County Roscommon, centered on the shores of Lough Key. The Mac Diarmada did not originally control it outright. The territory had earlier belonged to the MacRiabhaigh, or McGreevy, chiefs — a memorial stone at Ardcarne states plainly that the McGreevys were "kings of Moylurg" until 1255, a date that lines up with MacLysaght's own account of the McGreevys being "subdued by, and become tributary to, the McDermots" around the thirteenth century. From their stronghold on the Rock of Lough Key, later rebuilt as McDermott's Castle, the family ruled Moylurg as kings in their own right for the better part of four hundred years, functioning as both a military power in Connacht and a patron of Gaelic learning and the Church.
By the fourteenth century, the wider Mac Diarmada kindred had split into several branches. The senior line, MacDermot of Moylurg, held the overlordship from Lough Key. A related branch, sometimes called MacDermott Roe, or Red MacDermot, governed Tir Tuathail from a base at Alderford. A third branch, MacDermott Gall, held Artagh. All three descended from the same Mac Diarmada stock, but only the Moylurg line kept the kingship, and it is that senior line whose fortunes carry the family name into the modern era. In the sixteenth century the family also acquired substantial lands at Coolavin in south County Sligo, inherited through the O'Gara family, and it is Coolavin, not Moylurg, that gave the later chiefs of the name their lasting courtesy title.
How Did the McDermot Lordship Decline — and Survive?
Tadhg mac Diarmata, who ruled until 1585, was the last man to hold Moylurg as an actual kingship. His submission to the Composition of Connacht that year was part of a province-wide settlement under which Gaelic lords surrendered their claims to overlordship and military service in exchange for fixed rents and recognized freehold title under English common law — administratively tidy, but the end of independent Gaelic rule in Moylurg in any meaningful sense. Land confiscations followed over the following century, accelerating through the Cromwellian and Williamite forfeitures, and by the close of the seventeenth century the family had lost the core of its ancient territory around Lough Key entirely.
What did not disappear was the family itself, or its sense of its own standing. The senior line held on to property at Coolavin, and from the early eighteenth century its head began to be referred to, informally but consistently, as "Prince of Coolavin" — a title that stuck through popular usage among neighbors and genealogists rather than any crown grant. Charles MacDermot, head of the family from 1707 to 1758, is generally credited as the first to carry it in that form. The title has passed down through named heads of the family since, including Hugh Hyacinth O'Rorke MacDermot, who served as Solicitor-General and later Attorney-General for Ireland in the 1880s and 1890s — proof the family's public prominence long outlasted its kingship. Today the courtesy title is associated with Francis MacDermot, who succeeded his father Rory MacDermot on the latter's death in 2021. It carries no legal authority under the Irish state, but it reflects one of the very few Gaelic chiefly lines traceable in continuous, documented form from a medieval king to a living head of family.
Families researching this line today have plenty of ways to carry the name forward at home. A McDermot woven throw blanket puts the family name and Irish national tartan on a piece meant for everyday use rather than a shelf. For smaller everyday pieces, a McDermot heritage mug or a McDermot garden flag works just as well for marking a family home, a reunion, or a gift for a relative tracing the Moylurg line.
Rather than search product by product, the McDermot Irish Family Gifts & Heritage keeps every current McDermot item in one place, and updates itself as the range grows.
Is McDermot the Same Name as McDermott?
Almost certainly, yes — as a spelling variant, not a separate family. Consonant doubling is one of the most common inconsistencies in the anglicization of Gaelic surnames, and it shows up across many Irish names, not just this one: Sullivan and Sullivann, Fitzgerald and Fitzgerrald, Kelly and Kelley all have similar doubled-letter variants scattered through nineteenth- and twentieth-century records. Clerks, priests, and registrars working from spoken Irish or from earlier handwritten records were not applying a fixed standard, and a single family could appear with different spellings in the parish register, the passenger manifest, and the naturalization papers.
It is worth being direct about the documentary record here. Most published family history on this name — MacLysaght's entries, the pages on the Mac Diarmada dynasty, the succession of Princes of Coolavin — uses the double-T McDermott spelling, because that is overwhelmingly the form the senior Moylurg and Coolavin line itself has used. The single-T McDermot spelling this article is built around is a genuine, attested variant of the same Gaelic surname and family story, carried by households whose own paperwork settled on one T rather than two, whether through a clerical error generations back or a deliberate later choice. It is not evidence of a different Gaelic ancestor or a rival lineage — it is the ordinary unevenness of anglicization, visible in a single letter.
Where Do McDermots Live Today?
Within Ireland, the name remains most closely associated with Roscommon and the wider Connacht region, unsurprising given nearly four centuries of settled lordship there before the seventeenth-century confiscations scattered the wider kindred. Members of the family and its many branches also took root early in Sligo, through the Coolavin lands, and in neighboring counties as tenants, clergy, and later professionals moved beyond the old territory.
Like most Irish surnames, McDermot travelled heavily with nineteenth- and twentieth-century emigration, and today the name — in both its single-T and double-T forms — is established across the United States, Canada, Australia, and Britain. Diaspora communities with strong Irish-American or Irish-Australian identities frequently build family reunions and genealogical projects around tracing a McDermot or McDermott line back to a particular Connacht townland, a reflection of how well documented the family's origin point remains compared with many other Irish surnames.
Frequently Asked Questions About the McDermot Name
Is McDermot Irish or Scottish?
Irish. The name comes directly from the Gaelic Mac Diarmada, tied to a single Connacht family descended from the Mac Diarmada kings of Moylurg. It is not a Scottish clan name, though Gaelic naming conventions using "Mac" are shared across Ireland and Scotland.
What does McDermot mean?
It means "son of Diarmaid." Diarmaid was a personal name in wide use in early medieval Ireland, later anglicized separately as Dermot or Darby. As a surname, Mac Diarmada was fixed to one specific family line by the twelfth century rather than adopted broadly.
Is McDermot the same family as McDermott?
Yes, in the sense that matters for family history — McDermot and McDermott are spelling variants of the same Gaelic surname, Mac Diarmada. The double-T form is far more common in the historical record and is used by the senior Moylurg and Coolavin line specifically. The single-T McDermot spelling belongs to the same broader family name and story, differing by clerical or personal spelling choice rather than by ancestry.
Is there still a McDermot chief or prince?
There is a courtesy title, "Prince of Coolavin," that has passed down through named heads of the senior Mac Diarmada line since the early eighteenth century, currently associated with Francis MacDermot, who succeeded his father Rory MacDermot in 2021. It is a title recognized informally through tradition and genealogy rather than granted or enforced by the Irish state.
Where in Ireland does the McDermot name come from?
County Roscommon, specifically the former kingdom of Moylurg around Lough Key, with a later strong association with Coolavin in south County Sligo after the family acquired lands there in the sixteenth century.
Sources and further reading
Edward MacLysaght's entry on Mac Diarmada, hosted at Library Ireland, is the primary genealogical source for the family's founding ancestor and early branch structure. Background on the kingdom itself is drawn from the Wikipedia articles on Magh Luirg and the Mac Diarmada dynasty. Details on the courtesy title and its modern succession come from the Wikipedia article on the Prince of Coolavin, and on the wider legal transformation of Gaelic Connacht from the article on the Composition of Connacht. Biographical detail on Hugh Hyacinth O'Rorke MacDermot is drawn from his Wikipedia entry. Readers researching family records directly should consult the National Archives of Ireland's online census records for both McDermot and McDermott spellings in County Roscommon and County Sligo households.