By: Maurice Gleeson and Kyle DePew

Medieval Irish genealogies are the oldest in Europe, stretching back over 2,000 years. These were transmitted orally (prior to the introduction of the written record) so are likely to be prone to inaccuracies prior to about 600 A.D.

The first compilation of written records is believed to date from the mid-700s. Various versions of the medieval genealogies exist, and inconsistencies in these versions indicate further inaccuracies. Some of these appear to have been innocent transcription errors, whilst others may have been due to deliberate falsifications for social prestige or political gain.1

These known examples of inaccuracies have cast doubt on the veracity of the medieval genealogies as a whole, and this has been the subject of ongoing debate since the resurgence of interest in Irish heritage with the Gaelic Revival of the 1800s.

When FamilyTreeDNA made commercial Y-DNA testing available to the general public over 20 years ago, not only had they introduced a new tool for genealogical research, but they created the infrastructure that allowed genetic genealogy to emerge as a citizen science.

Y-DNA has since evolved into an incredibly powerful tool for the study of surnames and has the potential to shine new light on many old questions about these ancient Irish genealogies.

A new scientific article assesses what Y-DNA tells us about the medieval genealogies of the Uí Briúin (pronounced ee brew-in) dynasty of northwest Ireland.2 The authors are Maurice Gleeson, Kyle DePew and Bart Jaski. Maurice and Kyle authored this blog post and are project administrators of various FamilyTreeDNA Group Projects.

Bart is one of the leading academic experts on Irish medieval genealogies and was one of the guest speakers at Genetic Genealogy Ireland 2019, which was kindly sponsored by FamilyTreeDNA. (You can see his presentation here.)

Bart suggested that we should explore the link between the royal lineages of the O’Conor Don and the Mac Dermot of Moylurg, who reportedly shared a common ancestor in Tadg mac Cathail (pronounced tie-g mock caw-hull; born about 900 A.D.). The above article is the result of that work and is published in Peritia, the Journal of the Medieval Academy of Ireland. The article is aimed primarily at Irish medieval historians and it is hoped that it will enlighten this community about the usefulness of Y-DNA as an additional research tool, as well as the work that is being carried out by genetic genealogists (volunteer Group Project Administrators) into Irish clan research. Increasing awareness in this way may help foster further collaboration and cross-fertilization between the two disciplines.

The article has just been published and an e-version of the article will be available for download in May 2024.

Ancient Breifne Clans Ireland Group Projects Page

The Ancient Breifne Clans (Ireland) FamilyTreeDNA Group Project is one of the many group projects associated with the Uí Briúin dynasty.

Background: the precarious evolution of Irish surnames

Over the millennia, in a continuous process, Irish clans gave rise to subsequent clans. These descendant clans adopted specific surnames from about 900 A.D. onwards. Thus, the post-surname clans that descended from a specific pre-surname clan should be relatively closely related to each other genetically, and should therefore sit on adjacent branches of the Y-DNA Tree of Humankind (i.e., Y-DNA haplotree), probably with the same overarching DNA marker.

But there are many important factors that influence if a particular surname has survived over the past 1,000 years or so, and if there are living descendants visible in the Y-DNA database today. Some of the most important of these factors include the following:

  1. Extinction of surnames: some Irish surnames have gone extinct. Some will have died out, others will have “daughtered out” (i.e., no male heirs to pass the surname forward to future generations). The rate of extinction could be as high as 95%.3
  2. Survival of the fittest: some of these post-surname clans became quite powerful within the Irish social system of the time, whilst others did not. It is likely that the more powerful clans had more surviving descendants than the less powerful clans, and therefore there is probably a greater chance of the surnames of the more powerful clans persisting over time, escaping extinction, and being in the Y-DNA database today.
  3. Lack of testing: some surnames may indeed exist today, but none of the surviving descendants bearing that surname have done a Y-DNA test. And therefore the surname doesn’t appear in the Y-DNA database. Alternatively, some descendants may have tested, but not enough to provide useful DNA data for analysis. Even a well-run surname DNA project might test less than 0.2% of surviving bearers of the surname.4
  4. DNA switches (NPEs, or non-paternal events): some surnames may indeed exist today, but the surviving descendants bearing that surname have become “uncoupled” from the Y-DNA signature that was originally associated with it (i.e., an NPE or DNA switch has occurred). This could be due to a variety of different reasons (e.g., illegitimacy, infidelity, adoption). And so, even though the surname does still exist today, and may even appear in the database, it is no longer associated with the Y-DNA of the progenitor of the surname. This occurs in 33-55% of present-day descendants.5
  5. Surname switches: This is another factor leading to a reduction in the number of descendants bearing a particular surname. Again, there are many reasons for this, but in medieval times, an important factor was a family changing their surname to that of their chief or lord as a sign of allegiance, fealty, or loyalty. This process has been termed “surname gravity,” where less powerful families change their surnames to one associated with a more powerful family. In effect, less powerful surnames “gravitated” towards more powerful ones. Such a change might also confer a degree of protection on the family in question.5 Such surname switches would mean that the surname in question becomes associated with “foreign” DNA i.e., Y-DNA that comes from a source other than the progenitor of the surname.
  6. Anglicization of surnames: throughout the 16th to 19th centuries, Irish surnames were translated from the Irish version to an English equivalent. In many cases, the original Irish version was “lost in translation” and attempts to accurately correlate the current English version of a surname with the previous Irish version can be very difficult, or at times impossible.6 This is a particularly large-scale example of a surname switch—it affected an entire nation.
  7. Results are not made public: by default, Y-DNA results in the FamilyTreeDNA database are not publicly displayed on the webpages of Group Projects. Test takers have to manually reset this to allow public display but many people do not know this or how to do it. Secondly, if a test taker has not joined any Group Projects, their results will not be publicly visible. Thus, the data may be available, but inaccessible for research purposes.

Given the above, the chances may be very slim indeed that a particular surname from a particular progenitor is discoverable in the FamilyTreeDNA Y-DNA database.

The Uí Briúin dynasty of northwest Ireland

Around 2014, Paul Duffy from Dublin (genetic genealogist and Group Project Administrator) noted that quite a few surnames associated with the Uí Briúin dynasty of northwest Ireland fell under the SNP marker A259 (a descendant of M222).7 Since then, various FamilyTreeDNA Group Projects (run by volunteer Group Project Administrators) have explored this possible connection.

The specific surnames that are reported to be descended from the various branches of the Uí Briúin dynasty are summarized in Table 1, divided into those associated with more powerful (major) and less powerful (minor) descendant clans.

Each surname has a list of named ancestors (recorded in various versions of the medieval genealogies) that goes back to the reported progenitor of the Uí Briúin dynasty, namely Brión, supposed son of Eochaidh Muigh-Meadhóin (spelt in a variety of ways and pronounced roughly Yuck-ee Mwee Maydoyn).

Table 1

Surnames of the Uí Briúin dynasty
Some surnames associated with the various divisions of the Uí Briúin dynasty.

Note that each branch of the Uí Briúin dynasty gave rise to multiple surnames. Sometimes the same surname arose independently in several branches (e.g., McTiernan, Flynn, McDonough).

So, in relation to the surnames above, there are two key questions:

  1. Are there enough male descendants in the Y-DNA database to sufficiently reveal a genetic pattern (should one exist)?
  2. Is the genetic pattern consistent with the medieval genealogies?

In part two we will address these questions. This is the first part of a two-part series on the medieval genealogies of the Uí Briúin dynasty of northwest Ireland.


  1. Donnchadh Ó Corráin, Creating the past: the early Irish genealogical tradition. Peritia 12 (1998) 177–208.
  2. Kyle DePew, Maurice Gleeson & Bart Jaski, Tracing the Sons of Brión. The R1b-A259 Y-DNA Subclade and the Uí Briúin Dynasty of Connacht. Peritia 34 (2023) 9-45. DOI10.1484/J.PERIT.5.136859. Also available at
  3. Rob Spencer, personal communication, based on his Extinction Simulator (available at
  4. James Irvine, Y-DNA of a Scots-Irish Diaspora. Genetic Genealogy Ireland (2018), Belfast. Video presentation available at
  5. Maurice Gleeson. Goodbye NPE, Hello SDS – some causes of Surname or DNA Switches. Article available at
  6. Rev. Patrick Woulfe, Irish Names and Surnames (1923). Available at
  7. See 7th comment (by Paul Ó Duḃṫaiġ) in response to Debbie Kennett’s article at