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The O Mahony Surname

There are a great many O Mahonys in Ireland—the name is included, usually without the prefix O, among the hundred most common surnames. I t belongs almost exclusively to West Munster.  The vast majority of Mahony and O Mahony births are registered today in Co. Cork, the area historically associated with the clan. 


O Mahony chieftains were powerful, often described as princes. Their principal territory comprised the modern barony of Kinelmeaky and extended to the sea, with some fourteen castles on the coast of southwest Cork. The name O Mathghamhna—in modernized spelling O Mathuna—is derived from Mathghamhain (Irish for bear), the grandson of Brian Boru.  Mathghamhain was killed, with many more of the Desmond fighting men, at the Battle of Clontarf in 1014.  (Edward MacLysaght, Irish Families and Diarmuid O Murchadha, Family Names of County Cork)

The early O'Mahonys were Cenél nÁeda, princes of the ancient Eóganacht Raithlind.  For a time, they were kings of Munster and Desmond.  They take their name from Mathgamain, son of Cian, son of Máel Muad mac Brain, who was King of Munster from around 960 to 970, and then again from 976 to 978. From 970 to 976 he was king of Desmond. Cían's father was killed by Brian Boru at the Battle of Belach Lechta in 978 AD. However, Cian became a close ally of Brian Bóruma and married his daughter Sadb (Sadhbh, or Sadb, Saibh, Sadbh, Sadhb, Sive).  From this marriage descend the O'Mahonys. The yDNA that we are interested in is that which was transmitted from Mael Muad Mac Brian, to his son Cian, and from him to Mathgamain.


As an aside, and just to avoid confusion, Sadbh’s uncle was also called Mahon (Mathgamain). He was Brian’s older brother. It is possible that she called her son after him rather than after her father Brian. After all, to call him Brian after  the man who killed Mael Muadh would have been tactless. It is important to note too that Brian, and Mathgamain were “Mac Cinneide” and  did not have Eoganacht DNA. The Brian Boru DNA line can be found among the O Briens, Kennedys, and other names such as Hogan. DNA testing has highlighted that the Dal Cais claim of descent from Eoghan Mor was a convenience to add greater legitimacy to their claim to kingship.


The early territory of the O Mahonys comprised the modern barony of Kinelmeaky and extended to the sea. At one time they  possessed fourteen castles along or near the coast of southwest Co. Cork. The main clusters of DNA could justifiably be expected to be found among those who have origins in those regions.

Please note that in the name  Ó Mathúna (O Mahony), the Ó signifies Ua, meaning "descendant of,"  and it is important from the point of view of the DNA.  All male O Mahony descendants, regardless of race, ethnicity or the precise spelling of their last name, are welcome and encouraged to participate in the

O Mahony Surname DNA Project. The project analyses the Y-chromosome to determine kinship. For more information, please refer to the Genetic Genealogy portion of this site. 

Surname and Spelling Variants

With more than 1600 known spelling variants, the surname challenge goes far beyond the "with an 'e,' without an 'e,' with an apostrophe, without an apostrophe" debate.  Spelling variants can be found among siblings, much less from generation to generation.  Be diligent! 

O Mathghamhna     (derived from the given name Mathghamhnain ~1107)


O Mathamhna     (occasional early variant)


O Mahunus     (early Latin form in the medieval period, first non-Gaelic variant)


O Mahon     (Norman French or maybe an anglicization, most common form to ~1600, variants in the 1500s: Mahunde, Mahound, Maghound, Maghon, Mahoni, Mahone, Mahonie Mahowney, Mahowne, Mahowneye, Mahoone; also Ma[t]thew[s] used as attempted anglicization 1500s-1600s)


O Mathuna     (modern Gaelic form with variant Mhathuna)


O Mahony     (1600+; variants in USA of Mahoney, Mehaney, Mahny, Mahan, Mahn, Mahoon, Mahoune; additional variants on the continent of Mahanie, Mahono, Mahun, Mahuni, Mahuno, Maoni, Mauna, de Mahony)


Greater surname changes occurred in Ireland with a family of O Mathuna Ban anglicized to White, some Kearneys of West Cork may have been originally O Mathuna Ceithearnaigh, and a branch of O Mathuna became Canniffe.


For more information, see article “FORMS OF THE NAME” by Padraig O Mathuna, The O Mahony Journal, Vol. 14, 1990, pp. 23-25

Additionally, a December, 2003, O Mahony Society Newsletter article contributed by

Eileen Mahoney McConnell offered the following regarding surname variants found in the US Census:




Computer indexes are subject to an error rate of at least 15%.


It may be that the census enumerators did not understand the accents of our ancestors, and simply recorded a phonetic rendition of what he heard. The enumerations are all handwritten, and the indexer may have misread certain letters, for example, a “u” for an “n” in the indexes.

Mahoneys were in every State of the Union by 1850.


Abstracted from Accelerated Indexing Systems (AIS) indexes of all U.S. census indexes through 1850. 

Mahana     Mahenny   Mahorny   Mehenoy      Mahanaey   Maheny     Mahoun   Mehoney  Mahanah  Mahona     Mahuna     Mehony    Mahanahen  Mahonah    Mahunia    Mehorny  Mahanan  Mahonay  Mahunna   Mohanna   Mahanas  Mahone        Mahunny    Mohannah Mahanay  Mahonee  Mahuny    Mohanny   Mahane     Mahoney  Mahurne      Mohanue    Mahanee   Mahonrey Mahurney Mohany      Mahaney   Mahony     Mehanay   Mohoney     Mahanna    Mahooney Mehanny   Mohoneya Mahannah  Mahoony  Mehany    Mohony      Mahenna     Mahornay   Mehenny   Mahenney Mahorney   Mehenoe

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