The earliest Uí Sheasnáin are very much associated with the area along the lower Shannon between Killaoe at the southern end of Lough Derg and Limerick city, the lowest bridging point on the River Shannon. Research carried out by Dr Cathy Swift of the University of Limerick points to two possible origins for the Uí Sheasnáin family. The first (the evidence for which is found in the 17th century Leabhar Mór na nGenealach) points to a Muinntir Uí Sheasnáin (muinntir meaning people or clan) who were located in east Clare. This Seasnán family, we are told, was established more than a thousand years earlier by a descendant of Cas, the man who gave his name to the Dál gCais, the ruling dynasty of Gaelic north Munster. Seasnán lived in the 13th century and traced his origins to Cas who lived in the 4th century through Aengus (6th century), Carthann Fionn Og Mór (5th century) and Blad, a son of Cas.
Because this genealogy was being written in the 1600s, over a thousand years after the time of Cas, it may have been entirely spurious and invented to place the Uí Sheasnáin firmly within the greater Dál gCais ‘tribe’ with its associations with 11th century Brian Ború. There may, of course, have been some truth in the Cas origin story. The Dál gCais were very much associated with the O’Brien (Ó Briain) family but even if the Uí Sheasnáin and Ó Briain are descended from Cas, that is not to say that there would be a genetic link between the two groups as Gaelic clans were not organised solely on the basis of kinship.
The second origin story that Swift has put forward is that the Uí Sheasnáin are descended from a singular individual, Seastnán, associated with the Uaithne Thíre, a people from the hilly Sliabh Fraoich area on the Tipperary side of the Shannon immediately north of Killaloe/Ballina, near the modern village of Portroe in the mofern barony of Owney.
Whether the modern Sextons of the Clare/Limerick/Tipperary area are descended from either of neither of these origins is largely a matter of conjecture but advances in DNA testing may, in time, provide some further clarity. It does, however, seem plausible enough that at least one of these has some credibility.
The precise nature of the relationship between the Sextons of the lower Shannon region of Thomand and the Sextons of Cork and Cavan has also yet to be determined but it is possible that these branches came about as a result of the political turbulence of the 17th century when the old gaelic systems were well and truly ripped apart with the consequent displacement of tens of thousands of people from their former ancestral locations.
Surname mapping analysis carried out by Wilson shows that by the 1850s, the Sextons were found most commonly in Co. Cork followed by Clare, Limerick, Tipperary, Cavan and Kilkenny with much smaller concentrations found in Sligo, Kildare, Longford and Dublin. Much smaller numbers (often individuals) are found in a large number of other counties scattered throughout the country. It is possible therefore that at sometime (probably in the upheaval of the early 17th century), a number of Sextons moved south into Cork and north into Cavan. Why this happened is not known. The Sextons who moved south seem to have settled in the Donoghmore area of mid Cork and in the Timoleague/Barryroe area of west Cork. At a later point (perhaps the late 1700s), a number of Sexton families moved further west into the Skibbereen area.
As with most things in life, there is, however, an alternative theory. There may, in fact, have been three quite separate foundation stories for the three main Sexton ‘clans’ (Lower Shannon Sextons, Cavan Sextons/Tackneys and Cork Sextons). The three groups may have had three quite separate unrelated ancesters – all coincidentally known as Seasnán. This is not at all implausible. It is well known for instance that there are a number of quite distinct and unrelated Murphy families and O’Donoghue families.
Modern genetic research has shown that families associated with the Dál gCais show a particular genetic anomaly that is common and quite possibly unique to the O Briain (O’Brien) and other Thomand families descended from Cas. This variation is known as Irish Type III DNA (R-L 226 cluster). What is interesting however is that analysis of the DNA of Con Sexton (of the West Cork Sextons) shows that he does not share this genetic variation and in fact has Irish Type II DNA (R-M 269) which is most commonly found in populations indigenous to the south west of Ireland. So there are two possibilities. One is that the Sextons of Lower Shannon origin are not genetically descended from Cas (ie the 17th century genealogists were wrong) and therefore may or may not be related to the Cork Sextons . The other is that the Sextons of Lower Shannon origin may yet be shown to be genetically descended from Cas – but in which case they are not related to the Cork Sextons.