McMahon is an Irish surname derived from the historical Gaelic form Mac Mathghamhna, “son of Mathghamhain.” Irish surname scholarship records two major historic MacMahon families: the MacMahons of Oriel, centered in County Monaghan, and a separate Thomond family in south-west County Clare. Browse the McMahon & MacMahon Irish family gift collection, then read on for the story of both great families of the name.
What Do the McMahon and MacMahon Surnames Mean?
Patrick Woulfe records Mac Mathghamhna as “son of Mathghamhain,” with the personal name Mathghamhain glossed as “bear.” Older records show many anglicized forms, from M'Mahowna and MacMaghon to MacMahon. Mc and Mac are two spellings of the same Gaelic prefix meaning “son of,” so McMahon and MacMahon are one surname.
Why Are There Distinct MacMahon Families in Ireland?
Woulfe separates the surname into two major historical traditions. One was the MacMahon family of Oriel in Ulster, centered on County Monaghan. The other was the MacMahon family of Thomond in Munster, whose patrimony Woulfe places in Corca Bhaiscinn, corresponding to the baronies of Moyarta and Clonderlaw in south-west County Clare. Each family took the name from its own ancestor called Mathghamhain.
The MacMahons of Oriel and County Monaghan
Woulfe states that the MacMahons of Oriel rose after the decline of the O’Carrolls in the thirteenth century and retained the lordship of Oriel into the reign of Elizabeth I, with substantial possessions and influence in County Monaghan continuing into the seventeenth century. Modern research by Monaghan County Council likewise notes that the earliest references to Monaghan town date from the fourteenth century, when the county was ruled by the McMahon family and a principal residence stood on a crannóg at Convent Lake.
The MacMahons of Thomond and County Clare
The Thomond MacMahons have a royal pedigree of their own. Woulfe describes them as a branch of the O’Briens and traces their name to Mahon, son of Murtagh More O’Brien, King of Ireland from 1094 to 1119. Their historical territory was Corca Bhaiscinn in south-west County Clare. Woulfe also assigns Marshal Patrice de MacMahon, later President of the French Republic, to this Thomond family.
Are McMahon, MacMahon, Mahon and Mahony the Same Family?
McMahon and MacMahon are the same surname. Woulfe also lists Mahon and Mahony among the anglicized forms of Mac Mathghamhna, but Mahony belongs chiefly to Ó Mathghamhna, the O’Mahony family of County Cork. For family research, keep the spelling found in each record and pair it with a county, parish or townland.
McMahon Families in Monaghan Records
The surname remained well established in County Monaghan after the medieval lordship. The Dictionary of Irish Biography records Ross Roe MacMahon, born in County Monaghan in 1698, who became bishop of Clogher and later archbishop of Armagh. It also records Hugh MacMahon, born in 1660 into the family of Colla Dubh MacMahon of Cavany, County Monaghan, who likewise became archbishop of Armagh. Woulfe notes that three MacMahons in succession held the primatial see of Armagh in the first half of the eighteenth century.
How to Research a McMahon or MacMahon Family Line
- Start with place. Establish the most recent known Irish county, parish and townland. Monaghan is especially important for the Oriel tradition; south-west Clare is central to the Thomond tradition.
- Search spelling variants. Try McMahon and MacMahon along with older forms such as MacMaghon, and match dates, relatives and locations across records.
- Use the National Archives census collections. The 1901 and 1911 censuses can be searched by surname and place, while surviving pre-1901 census fragments and substitutes can sometimes take a line further back.
- Check civil and church records. IrishGenealogy.ie, the state's free genealogy site, provides access to substantial civil-registration and church-record collections. Compare parents, witnesses, occupations and addresses to link each generation.
- Build from documented generations. Once the tree reaches a parish in Monaghan or Clare, the history of the Oriel and Thomond chiefs brings the family story to life.