There is another clan called Uí Cléreacháin that originated in the western part of the ancient province of Míde (Meath) The name has been anglicised as Clerkin, Clarkin or Clarke and is today mainly found in County Monaghan.
The clan originated in the modern county of Westmeath which was in the centuries prior to the Norman invasion was part of the ancient province of Míde, the fifth province, literally meaning the middle in Gaelic. Míde was the territory of the High Kings centred at Temuir (Tara) held since the 5th Century by the Southern Uí Neill
The clan, part of the wider Uí Neill, were descended from Colman Bec brother of Colman Már and Aed Sláineare recorded in the annals as being lords of Caille Follamhain in the 11th century a place centred in the barony of Moygoish in the north of the county. However it has also been placed in some accounts as being in the parish of Killallon in the Barony of Fore in County Meath!
See www.libraryireland.com/topog/K/Killallon-Demifore-Meath.php
The Annals of the Four Masters records that in
The age of Christ 1018 ………..A predatory excursion by Maelseachlainn into the territory of the Fearaceall and a party of the army was overtaken by the FearaCeal and the Eli so that Domhnall uaCaindealbhain, lord of cinel- Laeghaire and Cass-MideMaelseachlainn’s lawgiver were slain; and Ua Cleircein Lord of Caille Follamhain was wounded and died after a short period….
In 1172 Henry II of England granted the kingdom of Míde to Hugh De Lacey to hold as a Liberty under the crown. De Lacey set about conquering the land and pushed northwards and westwards the Gaelic clans living there who did not wish to become subjugated. The Uí Cléreacháin moved northwards settling in parts of Airgialla and Breffni, modern day counties Monaghan and Cavan.
In the 1659 Census (see Irish manuscripts Commission) the name has been anglicised as O’Clerkan, O’Clerigan, and O’Clerican in County Fermanagh and as O’Cleregan and O’Clerian in County Monaghan.
By the time of Griffiths Survey circa 1852, listing the ratepayers in each townland, there are eighty three Clerkins, nine Clarkins, listed in Co Monaghan and found in every barony; only two Clerkins and two Clarkins in Co Cavan and three Clerkins and three Clarkins in Co Fermanagh.
The Clerkin/Clarkin name seems to have remained intact in South Ulster although like Uí Cléirigh there was probably quite a number who from the 17th Century onwards translated their name into Clark or Clarke.