The Clare MacMahons descend from Brian Boru through his great-grandson Muirchertach O’Brien. Their West Clare castles, the unrelated Monaghan MacMahons and which DNA test to take.
The Clare MacMahons (Mac Mathúna, “son of Mathúin”) are direct descendants of Brian Boru. They trace their line to Mahon, a son of Muirchertach Ua Briain, Brian’s great-grandson and King of Munster, who died in 1119. In the 12th century they took control of Corca Baiscinn in south-west Clare, and they ruled it from castles at Carrigaholt and Clonderalaw until the collapse of the Gaelic order around 1600.
MacMahon at a glance
- Irish form: Mac Mathúna (Mac Mathghamhna)
- Meaning: son of Mathúin, from a word for “bear”
- Descent: from Brian Boru, via Muirchertach Ua Briain’s son Mahon
- Territory: Corca Baiscinn, south-west Clare, split into Moyarta (west) and Clonderalaw (east)
- Not related to: the MacMahons of Monaghan (Oriel)
- Y-37: shows whether your paternal line fits the Irish Type III (R-L226) pattern.
- Big Y-700: places you on a specific branch of the tree, the test that project administrators recommend.
- Family Finder: covers all your family lines, not just the male line, and finds cousins from the last five or six generations. Women can take it too.
- Free projects: once tested, join the R-L226 project and the MacMahon surname project to compare results.
Origin of the name
The Irish personal name Mathúin (Mathgamain) comes from the word for a bear. It ran in Brian Boru’s family: Brian’s elder brother, whose murder in 976 set Brian on the path to power, was also called Mathgamain. Clare Libraries explains that MacMahon is one of the most numerous surnames in Clare, and that its holders may claim descent from Mahon, son of Muirchertach Mór Ua Briain, King of Munster, who was buried in St Flannan’s Cathedral, Killaloe.
Lords of Corca Baiscinn
In the 12th century Mahon’s descendants pushed into Corca Baiscinn, the peninsula of south-west Clare running out to Loop Head, and displaced the O’Donnells who had ruled it. They took the fixed surname MacMahon at about the same time.
When Donnchadh MacMahon, Lord of Corca Baiscinn, died in 1488, two rival MacMahons were set up in his place. From then on the territory was split in two: West Corca Baiscinn, ruled from Carrigaholt Castle, and East Corca Baiscinn, ruled from Clonderalaw. A 1576 deed fixed the boundary between the two lordships, and it survives in the modern baronies of Moyarta and Clonderalaw. The last inaugurated chief of the name fell at the Battle of Kinsale in 1602. The parish of Kilmurry McMahon still carries the family name.
Famous Clare MacMahons
- Máire Rua O’Brien (c. 1615 to 1686), the formidable lady of Leamaneh Castle, was born a MacMahon, daughter of Torlach Rua MacMahon, Lord of Clonderalaw.
- Patrice de MacMahon, Marshal of France and President of France from 1873 to 1879, descended from a branch of the Clonderalaw MacMahons that emigrated to France.
Monaghan MacMahons are a different family
The MacMahons of Monaghan, Lords of Oriel in Ulster, are a separate family with no Dál gCais connection. Both families are large and still concentrated in their home counties, Clare and Monaghan, so your ancestors’ county is the first clue to which one you belong to.
MacMahon DNA
A Clare MacMahon in the direct male line should test positive for R-L226 (Irish Type III), and should sit close to O’Brien testers on the Dalcassian tree, since both families branch from Muirchertach Ua Briain’s line. A Monaghan MacMahon will test into a different group entirely. Start with a Y-37 test from Family Tree DNA, then upgrade to Big Y-700 to compare yourself with documented O’Brien lines. For cousins on all your other lines, add Family Tree DNA’s Family Finder test. Our Brian Boru descendants and DNA guide explains what each result proves.
MacMahon places to visit
- Carrigaholt Castle: the MacMahon tower house on the Shannon estuary, near Loop Head. Carrigaholt is also a base for dolphin-watching trips on the estuary.
- Clonderalaw: seat of the eastern lordship, near Labasheeda.
- Leamaneh Castle, the Burren: home of Máire Rua. It is on private land, so view it from the road.
- St Flannan’s Cathedral, Killaloe: where Muirchertach Ua Briain, the MacMahons’ ancestor, was buried. Plan it with our Brian Boru trail.
Kilkee is the natural base for West Clare. The Cliffs of Moher are an hour’s drive north.
Related Dál gCais surnames
The MacMahons’ closest kin are the O’Briens. See also the McGraths, MacNamaras, O’Gradys, Kennedys, Hogans and Caseys. The Considines are another O’Brien branch, and one group of O’Deas carries O’Brien DNA. Or browse all Brian Boru kindred surnames.
Frequently asked questions
Are the MacMahons descended from Brian Boru?
The Clare MacMahons are. They descend from Mahon, son of Muirchertach Ua Briain, Brian’s great-grandson. The Monaghan MacMahons are not.
What does MacMahon mean?
Son of Mathúin, a personal name derived from the Irish word for a bear.
Is McMahon the same as MacMahon?
Yes. Mc is an abbreviation of Mac, and both spellings are used by both the Clare and Monaghan families.
Where did the Clare MacMahons live?
In Corca Baiscinn, south-west Clare, with their main castles at Carrigaholt and Clonderalaw.
Sources
- Clare Libraries Local Studies, MacMahon, MacMathúna.
- Michael MacMahon, The MacMahons of Clare, Clare County Library.
- Clare Libraries Local Studies, The Family Name MacMahon.