Sir,—You recently published an edited version of the winning essay in the 2025 Irish Chiefs’ and Clans’ Prize in Gaelic History by David Mac Donagh (HI 33.6, Nov./Dec. 2025, pp 16–19). The writers of this letter wish to raise some concerns relating to this paper, the approach to the research, the veracity of the points raised, and the seemingly single-minded conclusions drawn from the available data. Its very title, ‘Will the real Síl Muiredaig please stand up?’, encapsulates the tone and purpose of the essay, which seeks to assert that the Mac Dermots and therefore the Mac Donaghs are the ‘real’ Síl Muiredaig of Connacht and that the O’Conors are not.
We take issue with the paper in three main respects.
Firstly, the author equates genetic distance with what he considers ‘real’ sept membership, but neither he nor any of the rest of us fully understand the horizon between an ancestor’s DNA and his sept affiliation 1,000 years ago.
Secondly, the genealogy on which the author relies is demonstrably corrupt.
Thirdly, the trust which the reader should place in any piece of research must be tempered by the author’s use and acknowledgement of prior peer-reviewed studies examining much the same data.
DNA is a great tool for better understanding the past, but it is just one piece of a puzzle. The organisation and make-up of a powerful sept like the Síl Muiredaig was complex and, although genealogy was incredibly important, much more than DNA bound these septs together. So genetic distance does not preclude the O’Conors from being the senior line of the Síl Muiredaig.
David mentions infidelity, adoption and fosterage as possible explanations for an O’Conor surname/DNA switch, but doesn’t deal with how the genealogies were developed and the possibility that the genealogy of the Uí Briúin was politically shaped to integrate two powerful regional lineages into the Uí Briúin Aí and, ultimately, its most powerful sept, the Síl Muiredaig.
Genetically, the O’Conors are more remotely related to most of the other lineages of the Síl Muiredaig than the Mac Dermots but this is by no means unique. We see a similar situation with the O’Kellys of Uí Maine, the O’Neills of the Cenél nEóghain and the MacCarthys of the Eoghanachta. Each of these royal lineages are quite distantly related to vassal lineages that the genealogies suggest were more closely related. Serious work needs to be done to explore why this is the case but a DNA/surname switch by each of these lineages at the dawn of the surname era is perhaps the least likely explanation, given that these surnames have remained loyal to the DNA for a millennium since then.
With respect to David’s proposed alternative genealogy of the Uí Briúin, where the O’Conor line is said to be descended from a proposed second son of Muiredach Mál called Cathal, this source is corrupt and incorrect. David drew this pedigree from a Connacht King List compiled ‘from various ancient authorities’ by eighteenth-century antiquarian Peter O’Connell. Listed as kings 8 and 9 on this list are pedigrees which include Fergus and Cathal, supposed sons of Muiredach Mál. The alarm bells are rung by the absence of ‘Mic’ after Cathail in entry 9, which would explicitly identify him as a son of Muiredaig Mál. This is the only instance in the list of 58 pedigrees where the ‘Mic’ is omitted. This makes the King List problematical as a source and necessitates verification.
Cathal’s lineage can be found in Volume I of Dubhaltach Mac Fhirbisigh’s Leabhar Mór na nGenealach, translated by Nollaig Ó Muraíle. This appears under the heading ‘Kings of the seed of Muireadhach Muilleathan’. This tract firmly shows that the Cathail mentioned in the pedigree of ‘Maol Cathaigh mac Faghartaigh, mc Cathail’ was actually the son of Muiredach Muillethan (d. 702) and not Muiredach Mál of the fifth century.
Mac Fhirbisigh’s compilations should be judged more reliable than the compositions of eighteenth-century antiquarians but, if we were in any doubt, his tract is consistent with several entries in the Annals of Ulster (for instance AD 789, 818 and 872). It is clear that O’Connell, or his source, has somehow confused or interpolated the descendants of Muiredach Muillethan into the entry on the descendants of Muiredach Mál.
Finally, in respect of the background research undertaken to substantiate the claims of the paper, the reader firstly must find it jarring that two of the listed publications for further reading come from Wikipedia.org, a website which is not noted for its peer-review accuracy. No mention is made of two published peer-reviewed publications which deal in some detail with the differences between O’Conor and Mac Dermot DNA: K. DePew, M. Gleeson & B. Jaski, ‘Tracing the sons of Brión: the R1b-A259 Y-DNA subclade and the Uí Briúin dynasty of Connacht’, Peritia 34 (1) (2023), 9–45, and T. Fehér, ‘High resolution paternal genetic history of Ireland and its implications for demographic history’, Emania 26 (2023), 95–117. These would have provided some much-needed nuance to the conclusions put forward in David Mac Donagh’s paper, outlining the various alternatives as to why this was the case.
Suffice it to say, it is a little premature (and inappropriate) to ask the ‘real’ Síl Muiredaig to stand up. Labelling certain lineages as ‘real’ or ‘true’ members of a sept and eliminating others based solely on DNA oversimplifies things and doesn’t really get us any further forward. There is much more work to be done to fully appreciate the intricacies of the broader Uí Briúin genealogies and how these relate to their DNA.—Yours etc.,
PATRICK GLENNON
Tribes of Uí Maine Project
DR DANIEL P. CURLEY
Rathcroghan Visitor Centre
In reply to Mr Glennon and Dr Curley, I must correct a grave misapprehension. I am a Mac Donagh from the O’Flaherty cadet branch, not a Mac Donagh from the Mac Dermot cadet branch, so I have ‘no dog in the fight’ over who is the senior branch of the Síl Muiredaig, genetically speaking. I only wish to uncover and make known genetic facts that either corroborate or repudiate the genealogical records.
So far, the preponderance of the Dál Cuinn Y-DNA genetic data corroborates the majority of the Dál Cuinn genealogies. If anyone doubts that we are looking at the correct data for the Dál Cuinn, please see Tim McEvoy’s paper entitled ‘The Dál Cuinn (otherwise known as the Teóra Connachta and the Uí Néill): a geographical and genetic description of the R-DF104 clade’ in Journal of Genetic Geneology (2009).
My paper was written for an essay contest and the target audience was the general public. It was not intended to be a full-blown ‘peer-reviewed’ paper, but rather a general-interest paper showing how Y-DNA is being used successfully in genetic genealogy. I was constrained to using very limited attributions, and the Wikipedia article references were primarily used for easily understandable background information, as befits a general-interest paper.
Further, my paper was written in the context of Y-DNA patrilineal bloodlines. Of course, Y-DNA genetics cannot possibly provide any insight into political, cultural, or social matters. But the Síl Muiredaig BLOODLINE is the patrilineal descendants of Muiredach Muillethan. So, my statement that the later O’Conor lineages are not Síl Muiredaig genetically and that the Mac Dermot are the senior BLOODLINE branch holds, as is demonstrated by the Y-DNA genetic data, which my critics do not dispute. If a historical record calls for a bloodline relationship that is not corroborated by Y-DNA genetic data, then that genealogy is defective and must be questioned.
In that regard, they state that the genealogy I am using is defective, and then proceed to give the genealogy of
Muiredach Muillethan > Cathal > Fógartach > Máel Cothaid & Áed
as being the only correct genealogy. But there is absolutely nothing that precludes the approximately 200-years-earlier genealogy of
Muiredach Mál > Cathal > Fógartach > Máel Catha & Áed
from also being valid. They are assuming that only one of those can be correct. And the difference between Máel Catha and Máel Cothaid must be noted. But, as implausible as it may seem, it is completely possible that both genealogies are correct, especially if the earlier genealogy was known by the later family. We human beings are known for our propensity for doing implausible things.
In addition, the complaint about a single missing ‘mic’ after Cathal is slightly picayune since we aren’t referencing the source text and cannot be sure where it was dropped. Regardless, it is clear that a genealogy is being presented, and they raised no other objections to Peter O’Connell’s King List. However, it cannot be emphasised enough that the genealogy from Peter O’Connell’s King List is corroborated by Y-DNA genetic data while the Síl Muiredaig bloodline descent of the later O’Conor lineages is completely repudiated by Y-DNA genetic data. So the only ‘defective’ genealogy is the one that shows the later O’Conor lineages as being descended from Muiredach Muillethan.
This is a new field, and most historians that I have approached in the past have wanted nothing to do with science/DNA, and most geneticists had little interest in exploring historical analysis applications, preferring instead to work on medical applications. History can no longer rely strictly on source texts. It must employ a multidisciplinary approach using classical historical research, DNA genetic analysis, archaeology, linguistics etc. The use of Y-DNA genetic data to corroborate or repudiate historical patrilineal genealogies is a crucial first step in understanding our actual heritage.—Yours etc.,
DAVID C. MAC DONAGH