| Gaelic form | Mac Diarmada |
| Origin | Patronymic — descendants of Diarmait |
| Etymology | di (without) + airmait (injunction) — "free from prohibition," "free man" |
| Province | Connacht (primary) |
| Core counties | Roscommon, Mayo, Sligo |
| Historic territory | Moylurg (Magh Luirg), north Roscommon |
| Variant spellings | MacDermott, Dermott, Diarmott, McDermot, MacDiarmid (Scottish) |
The surname McDermott derives from the Gaelic Mac Diarmada, meaning "son of Diarmait." The personal name Diarmait — anglicised as Dermot — is one of the oldest and most storied names in Irish tradition. Its etymology is debated among scholars, but the most widely accepted interpretation derives it from the Old Irish elements di (without) and airmait (injunction, prohibition, or taboo), giving a meaning of "one free from injunction" or more liberally "a free man" — a name that carried connotations of personal sovereignty and independence in the context of early Irish society. The name was borne by one of the most famous figures in Irish mythology, Diarmait Ua Duibhne, hero of the great love tale Tóraíocht Dhiarmada agus Ghráinne (The Pursuit of Diarmait and Gráinne), and by one of the most consequential figures in medieval Irish history, Diarmait Mac Murchada (Dermot MacMurrough), King of Leinster, whose invitation to the Normans initiated their involvement in Ireland.
The Mac Diarmada sept of Connacht were among the great ruling families of the province. They held the territory of Moylurg — from the Irish Magh Luirg, the plain of the track — in the northern reaches of County Roscommon, a stretch of fertile and strategically significant land lying between Lough Key and the Curlew Mountains. From this heartland, the Mac Diarmada exercised lordship over a wide area of north Connacht, and their Rock of Lough Key — Carraig Mhic Diarmada, the island fortress on Lough Key — became one of the most evocative symbols of Gaelic lordship in the west of Ireland.
The McDermott name is most heavily concentrated in County Roscommon, the historical territory of the Mac Diarmada lords of Moylurg. The sept's influence extended into the neighbouring counties of Mayo and Sligo, and the name spread across the broader province of Connacht through the movement of families from the core territory over many centuries.
County Roscommon is the county most intimately associated with the McDermott name in Irish history. The northern baronies of Boyle and Frenchpark — the territory of ancient Moylurg — were the heartland of Mac Diarmada power for many centuries, and the ruins of their island stronghold on Lough Key remain one of the county's most evocative historical sites. Roscommon town itself was the seat of a powerful Anglo-Norman castle, and the county's history is one of sustained interaction and conflict between Gaelic Connacht culture and the encroachments of Norman and later English power. Throughout this long struggle, the Mac Diarmada remained among the most resilient of the Connacht Gaelic families.
Counties Mayo and Sligo, bordering Roscommon to the northwest and north respectively, received McDermott families through the natural expansion of the sept beyond its core territory. Mayo in particular, with its strong Gaelic Catholic culture surviving well into the modern period, preserved the McDermott name in substantial numbers. The great Marian pilgrimage site of Croagh Patrick in Mayo and the sacred landscape of Sligo — associated with the poetry of W.B. Yeats and with the mythological world of the Tuatha Dé Danann — formed the wider cultural backdrop against which McDermott families lived their lives through the centuries.
The Mac Diarmada held the lordship of Moylurg from at least the twelfth century, exercising a form of Gaelic territorial power that was simultaneously military, economic, and cultural. Like the other great Connacht families — the O'Connors, the O'Rourkes, the O'Flahertys — the Mac Diarmada fulfilled the role expected of Gaelic lords: maintaining order within their territory, providing protection and justice to their people, and acting as patrons of learning, poetry, and the church. The Annals of Lough Key, compiled by monks working under Mac Diarmada patronage at their island stronghold, represent one of the most important records of medieval Irish history from the Connacht perspective and are a testament to the cultural investment the family made in preserving their world in writing.
The Mac Diarmada of Moylurg, like the other Connacht Gaelic lords, faced sustained pressure from English expansion through the sixteenth and seventeenth centuries. The Composition of Connacht in 1585, negotiated under Sir Richard Bingham as the English crown's governor of the province, attempted to impose English landholding and taxation structures on the Gaelic families of Connacht in exchange for confirmed title to their lands. The Mac Diarmada, along with many other Connacht families, initially participated in this arrangement, but the broader collapse of the Gaelic order following the Flight of the Earls in 1607 ultimately undermined whatever security the Composition had offered.
The Cromwellian settlements of the 1650s, which confiscated vast tracts of land across Connacht and transferred them to English settlers and soldiers in lieu of pay, were particularly devastating for the Gaelic families of the west. Many Mac Diarmada families lost the lands they had held for centuries, and the impoverishment and displacement of the period set the conditions for the social misery of the eighteenth century that preceded the catastrophe of the Famine.
The most celebrated modern bearer of the Mac Diarmada name was Seán Mac Diarmada — John McDermott — born in Kiltyclogher, County Leitrim, in 1883. A key organiser of the Irish Republican Brotherhood and one of the seven signatories of the Proclamation of the Irish Republic read from the steps of the General Post Office in Dublin on Easter Monday, 1916, Mac Diarmada was executed by firing squad at Kilmainham Gaol on 12 May 1916. He was 29 years old. His name appears last but one on the Proclamation, and his contribution to the planning and organisation of the Rising was, by the testimony of his contemporaries, fundamental to its execution. Mac Diarmada was a native Irish speaker who had contracted poliomyelitis and walked with a limp — details that made his physical courage all the more remarked upon by those who knew him.
The McDermott name spread across the Irish diaspora in the great emigration waves of the nineteenth and early twentieth centuries. County Roscommon was one of the counties most severely affected by the Great Famine of the 1840s, with a population that fell from over 250,000 before the Famine to fewer than 175,000 a decade later through death and emigration. The McDermotts of north Roscommon and the broader Connacht region joined the vast outflow of the Famine years, with Boston, New York, and the industrial cities of the American northeast receiving the largest concentrations.
In America, the name became firmly established in Irish-Catholic communities across the northeast. In Australian history, the name appears consistently among the convict and assisted emigrant records from the 1820s onwards, with New South Wales and Victoria both carrying substantial McDermott populations by the late nineteenth century. In Argentina, a significant Irish community developed in the pampas region, and McDermott families from Roscommon and Mayo were among those who settled in the Buenos Aires area — one of the less well-known chapters of the Irish-Connacht diaspora.
McDermott research centred on County Roscommon is well served by several specific resources. The Roscommon Heritage and Genealogy Society maintains a significant database of county records and can be particularly helpful for families with roots in the north of the county — the territory of historic Moylurg. The Strokestown Park Famine Museum, associated with the Mahon estate from which thousands of Roscommon tenants were evicted and assisted to emigrate to Canada in 1847 and 1848, holds material relevant to families from the area.
Civil registration records from 1864 and Catholic parish registers for Roscommon are accessible through IrishGenealogy.ie and through the National Library of Ireland's microfilm collections. Griffith's Valuation from the 1850s is fully searchable at AskAboutIreland.ie and provides invaluable townland-level data for McDermott families in their core territory. The 1901 and 1911 census returns are freely available from the National Archives of Ireland at Census.NationalArchives.ie and are essential for identifying surviving families in the early twentieth century.
Love Ireland covers the places, townlands, and stories behind Ireland's great surnames — written for the diaspora, by people who know the landscape.
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