Surname yDNA Project

Project launched 1 August 2003 < Page revised 25 September 2010 < Page updated 9 February 2011


Origins of 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 commonest surnames.  It 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)


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.  Men inherit both their Y-chromosome and their surname from their fathers; and pass both on to their sons.  Women do not have a Y-chromosome, and traditionally adopt their husband’s surname.  Therefore, female descendants are only able to participate in the O MAHONY Surname DNA project through a male O’Mahony relative.





Project Background


The O MAHONY Surname DNA project was begun on 1 August 2003 by Perry Streeter and was transferred in early 2007 to the O Mahony Society for ongoing administration.  It is currently administered by Dr. Finbar O’Mahony.


Geneticists can now study the Y-chromosome DNA of males bearing all variations of the O MAHONY surname to determine whether or not they are related and the approximate degree of their kinship.  For an excellent explanation of this process, please see DNA 101 at the BLAIR DNA Project.


Using an ethical and competent testing company, Family Tree DNA, we are creating an O MAHONY Surname DNA database.  To address any concerns that you may have about privacy, please click here.  To collect a DNA sample, a simple, painless cheek-scraping test is used instead of a blood test. View a video of cheek swabbing process.


Be aware that DNA testing can also reveal secret adoption or maternal infidelity anywhere in a lineage.  Consequently, any O MAHONY male who tests his DNA should be prepared for the small possibility that he may not be an O MAHONY, genetically speaking.


Project Objectives



We are especially seeking participants who are descendants of the O'Mahony Clan's major septs/branches (view list with map). If you have credible family lore and/or documentation linking to one of them, financial assistance may be available. Please email Dr. Finbar O’Mahony for more information.



Project Results


Initial test results for this very small sample suggest that all O MAHONY descendants do not share a common ancestor as several of the columns for distinct markers have different values indicating a genetic difference between the individuals. To see the various reports summarizing the results go to


            O’Mahony Surname Project Participants’ Results Reports



Go to “Quick Interpretation Guide” for help understanding the technical details of how to derive meaning from differences in the numbers found in the tables.


A larger number of participants are needed before we expect to see more distinct family clusters forming. In time we hope to identify members of each cluster who are documented as being from specific areas of Cork and Kerry. Join now.



How to Participate in the Project


If you are a direct-line male O MAHONY descendant and would like to have your DNA tested for matches with others, simply click on the following link…



            O’MAHONY Surname Project at Family Tree DNA



            As a project participant, you will be eligible for a discounted test price of:

        $239 for a 67-marker test (ideal, one can upgrade later for additional $$)

        $149 for a 37-marker test (recommended)

          $99 for a 12-marker test (discouraged, used primarily for variant spellings to rule out O’Mahony link)

plus a small fee for shipping & handling and return postage.



How to Donate to Sponsor Others

Please contribute to sponsor participants the DNA Committee believes will help us reach our project objectives. You can use a major credit card via PayPal and do not need a PayPal account to contribute. Go to:

and select the letter “O” in the left box and then “OMahony” in the right box above the donation information boxes which you should fill in and then click on the yellow PayPal button. It will take you to the webpage for your credit card information. Thank you.