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Join us at our 2023 Gathering for our "Explore Your O Mahony Roots" seminar, which will highlight pan-genealogic research and tips!

Points of interest will include: 

Ta failte romhat


Welcome to the O Mahony yDNA project update. 


The y-DNA Project started in 2003 and continued under Dr. Finbar O Mahony’s guidance until his passing on May 22, 2022.  The current Administrators are: Patrick O Mahony, Tom Weingart and John W. Mahoney.  We miss Finbar for many reasons including: his long term commitment to the Society’s Y-DNA Project, his understanding of the historical relationship between Irish people and events, and as you will soon hear – his ability to correctly pronounce Irish names and places (a talent I have not yet acquired).   I also miss his sense of humor and his gentle tolerance for my many questions.   He was a very fine gentleman.


How did the O Mahony name originate? 


The O Mahony’s are named after Mahon (Mathmahana) Chief of the Eóganacht Raithlind, who died sometime around 1038.  Mahon was the son of Cian who led Brian Boru’s 2nd Division at the Battle of Contarf.  Mahon’s mother was Sadhbh, daughter of Brian Boru.   


At that time, the name Mahon was a common given name or forename so that there could be many O Mahony families named after a Mahon that are not related to Mahon mac Cian. 

For example, Brian Boru’s elder brother was named Mahon and is believed to be the ancestor of the MacMahon clan.  So the common use of the Mahon given name may be one reason among many that may account for the diversity of family lineages we see in the Y-DNA project.  But there also other explanations, these include: the welcoming nature of the tribal community at that time, an NPE, adoptions, men taking their wife’s surname, name changes to further a rise to notoriety, or as Finbar once said to me about my ancestors, some people changed their name to help hide from the law.


So what have we found about the yDNA makeup of the members of the project?  



Overall, there is a large degree of diversity within the project.   However, a number of clusters or branches are evolving and I’d like to highlight a few of those in my remaining time.  



CTS4466 (Irish Type II)


The largest cluster in the project has 74 members and matches a recognized type called Irish Type II.  Irish Type II was first identified by a specific STR pattern but is now associated with a defined SNP,  the CTS4466 SNP.  Irish Type II is a very common haplogroup in southern Ireland and so it is logical that this would be a significant cluster in the project.  Of the 74 members in this cluster, 22 or 30% have done a Big Y test.  This means many SNPs downstream from CTS4466 have been identified and it is becoming easier to see the degree of association among the cluster members.  


Almost all of the cluster members have O Mahony or variant surnames and their earliest known ancestor (for those who know it) originated in Ireland, in areas historically associated with O Mahony homelands.  Based on SNP testing many members of the cluster have a common ancestor dating to the time when the O Mahony clan was active, though they don’t all share that same ancestor.  That is, there are multiple family lines within the CTS4466 cluster.  


So are any of these men the direct descendant of Mahon mac Cian?  While it impossible to connect anyone with a historical figure like Mahon these members do appear to have ancestors that lived in the general time frame and in the same geographic area of the O Mahony tribal chiefs.  




L226 (Irish Type III)


A second cluster in the project is associated with Irish Type III.  As with Irish Type II, Irish type III was originally based on STR patterns but is now associated with the L226 SNP.  We have 17 members in this group.  Five of the seventeen, or 29% have taken a Big Y test.  The L226 SNP is associated with Brian Boru who was a member of the Dalcassian tribe.  This is important because while Mahon mac Cian is a grandson of Brian Boru, his connection to Brian Boru is through his mother and not his father. Mahon mac Cian’s yDNA is Eóganachtan, not Dalcassian.  So why do these O Mahony’s have a Dalcassian SNP?  There are various suggestions, one is that Brian Boru’s daughter brought some kinsmen with her when she married Cian and the kinsmen became O Mahony’s.  Another is that these O Mahony’s are actually descended from Brian Boru’s elder brother who was named Mahon.  Gwynneth Bennet suggests in her paper (see Acknowledgements below) that members of the Dalcassian and Eóganacht tribes were living and warring in the same area and in an attempt to assert their rights to Eóganacht lands, the Dalcassians may simply halved changed their name to O Mahony, a well know Eóganachta name.  Whatever the reason the L226 cluster is especially interesting because there is both definitive DNA evidence of a close relationship among a subset of this cluster as well as historical documentation that these O Mahonys have lived in the Mitchelstown area for a great many generations.  In genealogical research it is rare to have both DNA and historical documentation, and for them to support each other.   


(See Gwynneth Bennet’s 2023 O Mahony Journal Article, referenced in




M222 (Irish Type 1)


We have a very small cluster of two members associated with Irish Type I.  Like Irish Type II and III, Type I was originally based on an STR pattern but is now associated with the SNP M222.  At one time it was though that all men with the M222 SNP were descendants of Niall of the Nine Hostages, King of Tara, but that belief is now being  questioned.  However, the M222 SNP is very common in Northern Ireland and Scotland.  The two men in this group are very closely related and share a common ancestor around 1800.  Both of our two members know their earliest known ancestors were from western Ireland (Bantry and Crookhaven) and for one member there is a strong family tradition that his paternal ancestor was known as O Mahony Finn. Thus this group may be part of the O’Mahony Finn sept that was part of the Western O Mahony homelands.




The last group to be highlighted today is the FGC5494 cluster.  It does not have a

“type” designation and was only recently identified in 2014.  There are 25 members in this cluster with five members or 20% who have taken a Big Y test.  FGC5494 has a broad distribution throughout Ireland and the European continent and it is difficult to associate this haplogroup with any historical O Mahoney homeland or tribal chief.  If one maps known locations for the earliest ancestors for the members of this cluster you find locations spread throughout the traditional O Mahoney homelands from Cork to the Mizzen Head Peninsula as well as Northern Ireland and Belgium.   As more members join or upgrade to higher marker tests a clearer picture for this group should emerge.  


And this last comment could be said for the entire project.  As more people join and more SNPs are identified the O Mahony project data will get more refined with a greater understanding of our relationships to each other and to the history of the O Mahony tribe.  




  • Many thanks for the work and dedication of Dr. Finbar O Mahony in his support of the Y-DNA Project.


  • Gwynneth O’Mahony Bennet’s - O Mahony Society 2023 Journal Article:

“O’Mahony by Name: Dalcassian by Decent”. 



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