McMahon Surname DNA Study


Many of us whose ancestors emigrated from Ireland are uncertain where we came from - either County Monaghan or County Clare, the two places in Ireland where the Mac Mahon surname arose.  But our ancestors have sometimes come from Dublin, or Scotland or perhaps emigrated elsewhere, to Europe or Australia.  There are also two or more separate septs of Mahon who originate in Ireland and may now be known as McMahon in the US or elsewhere and there are variations of the McMahon name, such as Mathews, McArdle (named after Ardle MacMahon), McPhillips (named after Phillip MacMahon) and Ennis and Connolly.  Some individuals changed their names or dropped the Mac for various reasons pertaining to peculiar events in Ireland at  the time.

We have on our site the MacMahon genealogy for the Monaghan MacMahons from the time of the Collas up to the 1640s and in some cases have been able to construct additional family lineages beyond that decade.  But due to many events pertaining to turmoil in our homeland there are essentially no records between the 1640s and the early 1800s, leaving us with a nearly 200 year gap to fill.  This is most often impossible to accomplish.

Many descendants of émigrés then do not know whether their roots are with the Ulster sept of MacMahons in County Monaghan or with the Clare sept or one of the Mahon septs. 

One of our curious cousins has launched an effort to find out and I've agreed to aid that effort by posting the information on the website.

Basically, the study is conducted by Family Tree DNA, a reputable firm that provides DNA testing.  They are based in Houston Texas.  You can sign up online or mail a check.  The study tracks the male Y chromosome and only males with the surname McMahon should sign up.  If you're female and interested, please get a male member of your family to sign up.  Also, it doesn't hurt to have multiple participants from  your family as this provides for greater certainty in the results.  The company requests two participants from each family line.  I'd suggest these be cousins rather than brothers, even distant cousins.  There's no need for two brothers or a father/son to participate as their dna will be the same or nearly so.

When you go to sign up, be sure to request the 25 marker sample at a cost of $148.  Here's why:

1.         Western Europe is so non-diverse, genetically, having been settled recently, and by just a few actual founders, that we needed 25 markers to clearly separate families from the ‘background noise’ of people related, but within the time frame of 2500 years not just 300-500 years.


2.         The extra 13 markers act as a refinement when 2 people match 12/12 or 11/12 it is helpful to see whether they converge or diverge with the more volatile last panel of 13 markers…if you ’hang together’ on the refinement then the relationship is pretty well established, but if you diverge on this second panel then the relation isn’t.

So, basically the cheaper sample ($99) will only tell you that you are of European descent.  We're looking for greater detail than that, wanting to know which clan we come from.

Here is the site:

Join the MacMahon DNA Project

Be sure to sign up for at least the 67 marker test.  Following that, a test kit will be mailed to you and you'll swab the inside of your cheek and send the swab back to the company where they will be able to extract the information they need.  The results will be available four to six weeks after they receive the completed kit back from you.

As a participant you will receive a personal page on the website that shows your matches, thus you will learn who you are related to.

Here's what we know so far:

You may view all of our dna results here:

Here is a paper describing what the dna project has learned to date:

It is now very clear that we can distinguish between the clans based on their dna.  It is also clear that they are not closely related.

Anyone joining the project in the future can tell which clan they descend from.


WA McMahon kit 13852 and RF MacMahon kit 14876 (note the variation in surname) match on 21 of 25 markers and one might conclude therefore that they are not related.  Four dna markers have had an opportunity to evolve since their most recent common ancestor.   They likely are related because of the information provided by P McMahon below, though perhaps some 750 years ago.  In other words, since they are both MacMahon and both match P McMahon to some extent though they are distant from one another, they are not likely to descend from some other sept of McMahon.

P McMahon kit 14263 is one of our known County Monaghan Ireland anchors of the Farney branch of MacMahon, meaning he lives there and we know he's a Monaghan Mac Mahon.  Note that his first 12 markers match RF exactly, but miss WA by one (# 439).  So, WA McMahon and RF MacMahon are originally from Monaghan.  Their families though have been away for a very long time.  These McMahons match people named Boyce (Bogue in Gaelic, a  numerous family in the Fivemiletown area of County Tyrone), and two Americans named Hughes and Neal.  These would be Hughes and O'Neill of Monaghan and Tyrone and parts north, or the northern Ui Neill tribe or the O'Neills of the Fews, an area of south Armagh.  The MacMahons and the O'Neills intermarried frequently but for the dna to match closely the O'Neall male line would have to have taken over from MacMahon.  In other words, if MacMahon were of the Airghialla tribal origins but now matches O'Neall, then an O'Neall male would have had to have parented a male MacMahon child who retained the name MacMahon and from whom Monaghan MacMahons now descend.  A rather delicate topic, but quite possible.  Either that, or the MacMahons have always been of Ui Neill tribal origins and their historical pedigree is wrong.  We've already described elsewhere on this website the known falsification of the MacMahon pedigree.

T Mathews kit 14096 is supposed to be a Monaghan MacMahon from Dartrey but he differs from P McMahon by 3 markers (#s: 390, 385b, and 439) of his first 12 and 7 markers overall.  It would appear that they are unrelated.  Interestingly, T Mathews is a closer match to the McManus and Maguire clan of Fermanagh, matching a number of  individuals exactly (results not posted above).  Both McMahon and McManus are said to descend from the Collas but dna tests do not support this assertion.  Note the 11, 14, 12 markers in 385 a, b and 426.  T Mathews is also the only one of the group that has a 25 in marker 390.  These markers all match the McManus clan (not illustrated).  Phil McManus has advised us: "In the Maguire lineage (descendants), there is a MacMahon sept which derive from Mahon, grandson of Donn Carrach (not Don Mor) Maguire.  We McManuses share the same Maguire derivation."  This means that there may well be (or is) a second McMahon sept in Airghialla, of Fermanagh origin and not descended through the line of Niall Mac Mathghamhna. 

Interestingly, this second MacMahon sept, descended from the Maguire clan, now shows up in G MacMahon, kit 26675.  G MacMahon matches T Mathews, thus confirming that Mathews was a derivation of MacMahon and confirming the existence of this Maguire sept of MacMahon.

T Mathews is clearly related to the Maguire clan and not MacMahon of Monaghan.  The other explanantion is that T Mathews and G MacMahon represent the actual dna line of the MacMahons from the Collas and the other, larger line of Monaghan MacMahon descend from another chieftain who took the name.  There's no way to know at this point.

We have, however, confirmed the existence of an entirely different sept of Mac Mahon of Fermanagh, descended from Maguire and we've added their lineage here: Clann Mahon Maguire.  Frankly, this is a valuable contribution to history directly attributable to our DNA Project.

There is also a great deal of genetic variation within what appears to be the Monaghan sept.  This could be that there are more origins of the clan than history suggests, other families took on the MacMahon name, or that genetic change occurs much more rapidly than suggested by the geneticists.     

Regarding MacMahon, we are looking at two different septs of McMahon or Mahon above, with two very different dna samples.  As additional samples come in from other McMahons I'll break these out into groups so we can begin to see who is related to whom and look for any patterns that emerge.

MP McMahon Kit 14151 and H Mahon kit 15092match on 22 of 25 markers and are related, but the date to their most recent common ancestor (mcra) may be as far back as 750 years.  One might question their relationship with three markers off, but J Mahon provides the link:

J Mahon kit 14150 of Cooldaragh is known to be related to MP McMahon, with a common ancestor of John Mahon in 1845.  Interestingly, they do not match on all 25 markers but differ on marker 449 where it appears that MP McMahon has evolved away from J and H Mahon.  We tested both J Mahon and MP McMahon in order to have a duplicate sample to compare to H Mahon (who claimed to be totally unrelated) and to the Monaghan MacMahon clan.  MP McMahon is my brother.  What J Mahon actually provided us was the evidence that it was MP McMahon that had moved away from the other two on one marker.  J and H Mahon differ by two markers.

Our Mahons are related to JJ Mahon, kit 18480 with a 97% certainty but as far back as 1500 years ago.  This JJ Mahon does not know his origins.  My guess is Ulaid in the north due to the surnames he matches, but it could also be the area west of Tullamore.

As it is, there are very distinct patterns shown above.  Note the difference in marker 390 and the 11,11,12 combination in 385 a, b and 426 for H and J Mahon and MP McMahon.  What we have here is a very rare combination of markers, with no other surname matches in Ireland at present.  This suggests a very small sept or a branch of Mahon or MacMahon that has been largely eliminated with few survivors.  As you can see though the dna markers are a very different pattern than any of the others above.  Bennett Greenspan at Family Tree DNA suggests that this 11, 11, 12 combination is an originating dna marker, meaning older than what are more common combinations today of 11, 14 or 11, 15.

Note the 11,15,12 combination for these same markers in WA, RF, and P McMahon.  Also, look at the pattern in the last 4 markers on the right.  There are other differences as well, but these seem to be emerging as important markers for us to observe.

In P, WA, and RF McMahon we have confirmed Monaghan MacMahons.  What we do not have is a match to T Mathews or McManus.  In addition, studies at Trinity College in Dublin seem to indicate no common ancestor between McMahon and Maguire.  These comments too are based upon limited samples but it would appear that the MacMahons of Monaghan do not descend from the Collas as our story suggests.  If they do, and anything is possible, then a change of 7 markers has taken place indicating a very long period of genetic isolation from Mathews, Maguire, and McManus.

There is greater similarity between MacMahon and O'Neill, again based on a few samples.  Enough to warrant further study.  Is it possible that Niall Mac Mathghamhna was sent to Airghialla by O'Neill to take charge after the murder of O'Carroll and to provide a buffer between O'Neill and the Norman forces?  An interesting question that would explain a great deal about Niall's presumed authority.

This however raises another question in that the Ui Neill and Airghialla are, according to written history, related to one another.  The dna samples suggest otherwise, but again further analysis is required to draw such a conclusion.

On our public page listed above we now have an even greater number of genetic variations.  When the names vary greatly from those we have found to be anchors for our study, we can either take a guess at the person's origins, based upon similarities to dna markers of other participants or we can wait until we gain more participants.  If you are a participant and fall into this category, bare with us.  My own family is waiting for more participants to learn our origins.  I have my own theories, but that's all they are, straw dogs that I put up that give me an avenue to further research.  Only time will reveal other facts.

In kit # 30752 we have a MacMahon of known ancestry from Monaghan but with Viking dna.  In kit # 16690 we have a MacMahon with dna markers from a small clan in the Balkans that migrated to Spain long ago.

In kits 25806, 19007, and 20797 we have individuals who have not been linked yet to a particular sept.  These may represent the Clare sept.  Only participation by a known Clare MacMahon can help us to clearly identify members of that clan.

And Don Schlegel offers the following caution: "... seems like a word of caution is needed regarding any historical conclusions. To reach them, one has to assume that a man's father always was the person the mother said he was -- or simply didn't say otherwise.  Even if this were true in 49 out of 50 generations (or 1500 years), that one difference would throw the whole dna study off.  Likewise, adoptions, of course never recorded back then, would produce different genes in what legally and culturally and affectionately (and politically for the important families) is the same family."

Reminds me of the quote: "Oh what a tangled web we weave when first we practice to deceive."

Possible explanations for dna markers in a clan that vary from what one might expect in light of historical accounts include:

1) Extra-marital event
2) Fosterage
3) Major divergence... statistical mutation is merely an average of expected diversions.  It does not rule  sudden mutation in a short number of generations.  It does not take into consideration mutation of material due to diet changes or those due to environmental changes.
4) Fact differing from traditional accounts due to inaccuracies.
5) Fact differing from traditional accounts due to downright falsehood or fabrication.
6) Confusion of families.

As we learn more, we will share more.  This certainly is an area that warrants funding and further study, perhaps even a doctoral thesis comparing dna samples to the written history of Airghialla.

We need more Clare McMahons to join this project.  You will be helping others who do not know their ancestry. 

We are making history.  Join us!


For information on the validity of DNA testing you can read the following article:

Here is another article that explains how dna testing identifies relatives:


Family Tree DNA Notes:

It is obvious from our observation of 1000's of samples that some markers change or mutate at a faster rate than others. While that actual 'faster rate' has not yet been definitively calculated, not all markers should be treated the same for evaluation purposes.

The markers in red have shown a faster mutation rate then the average, and therefore these markers are very helpful at splitting lineages into sub sets, or branches, within your family tree.

Author's notes:

For those of you with any doubt whatsoever as to your roots I will tell you this: it may well be impossible to trace your ancestry beyond 1820 or so.  By participating in this study you will know your roots with certainty and then will know the genealogy of your family from 1640 and back in time as those lineages have often been recorded.  Join us!

I would not be recommending this if we did not see it as viable.  Though we continue to research our roots for persons missing in the timeframe of the late 1700s we are participating in this study in order to confirm our relationship to the MacMahons of Monaghan.  I would encourage you to do the same.  This type of study may provide your only option for confirming which sept you come from.

We receive no compensation from this study or for referring you to this study.  The study is done solely for the purpose of aiding all of us in determining our roots.  The more who join, the better the results will be.

We look forward to the results of an Ulster surname study being done by Trinity College in Dublin. 

For questions, you may me or Patrick McMahon , who is a genealogist in Ireland.

You may also wish to join the McMahon email list at RootsWeb.  To join the McMahon list, send an email to MCMAHON-L-REQUEST@ROOTSWEB.COM  and placing SUBSCRIBE in the message.

~ Jim McMahon

January 13, 2006

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