Page 17 - O Mahony Journal 2025

• 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 haplotype in southern Ireland, so,
logically, 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 that 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 mac Cian, these members do appear to have common
ancestors that ived n the general time frame and the same geographic area of the O Mahony tribal chief.
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• 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. That is, Mahon mac Cian’s yDNA is
Eóganachtan, not Dalcassian.

So why do these O Mahonys 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 Mahonys. Another is that these O Mahonys are actually descended
from Brian Boru’s elder brother, who was named Mahon. Gwynneth Bennet suggests in her
2023 O Mahony Journal Article 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 have changed their name to O Mahony, a well-known
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.

• 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 thought that all men with the M222 SNP were descendants of Niall of the Nine

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