As has already been stated, the earliest mention of the name MacClerken is in the Annals of Ulster for the year 1012, with the recording of the death of Scolaighi Mac Clercen, priest of Armagh. Scolaighi died two years before Brian Boru was brought to rest at the Cathedral in Armagh, whose death is described this way in The Annals of Ulster: "An affliction of the colic in Ard Macha in the above year (AD 1011 alias 1012), and a great number died of it. Mael Brigte son of the smith, lector of of Ard Macha died of it, and Scoliage, son of Cleirchene (MacClerchin), priest of Ard Macha...died."

The next mention of the name is in the Annals of the History of Ireland, for the year 1013: Cairbre MacCleirchen, Lord of Ui Fidhgeinte[died]. (A note in the Annals of the Kingdom of Ireland, edited by John O'Donovan, Dublin, 1854, says,"CLEIRCHEN--He was the ancestor of the family of O'Cleirchen, now pronounced in Irish, O'Cleirachain and anglicised Cleary and Clarke, a name still extant in the County of Limerick.) Ui Fidghenti was a large tract in the County of Limerick. Whether Scolaighi (Scolar) MacClercen and Cairbre Mac Cleirchen were brothers is not clear, but it seems likely that Scolaighi Mac Clercen, as priest of Armagh, was of a family in the general region of Armagh, which was an hereditary See where positions in the Church were inherited by sons. At that time, clergy could marry, as in the Orthodox Church today. The Catholic Church in Ireland did not become fully "Roman" until the twelfth century. It is possible that Scolaighi's father was Clercen, who was the son of Leran and who died in 995. Clercen himself was priest at Armagh. His father , Leran, may have been a member of a ruling family near Armagh. The practice of church positions being hereditary ended in Armagh about 1178.

There are other Clerkins names in the Annals who were possible, but not likely, antecedants of the MacClerkens. Cleirchen, m. Murchad, king of Clare-Galway, who died in 912;Cleirchen m. Conallan, airchinnach Daire Calcaigh (Derry) who died in 950; and Cleirchen m. Donngal, abbot of Fore in 981. If Cleirchen m. Mael Duin, who was mentioned earlier, was Scholaighi's father, this would explain the description of Scholaighi as a "noble priest of Art Macha" in the Annals of the History of Ireland, since Mael Duin was king of the Ui Echach. It is credible that Cleirchen m. Mael Duin, was the Clerchen who was the father of Scolaighi. However, it is not possiblle to say with certainty who Clercen's father was, but it is possible to say that Clercen's family almost certainly lived in the region of Armagh.

For the year 1018 Ua Cleircein, Lord of Caille-Follamhaim, was wounded and died after a short period.

Other O'Clerkens and MacClerkens are mentioned in the Annals. One who was a scholar and a teacher is mentioned for the year 1043: Cellach Ua Cleircinn, "successor of Finnen and Mocholmog...died on pilgrimage at Art-Macha." (Ard Macha, now Armagh, was founded by St. Patrick as his See.) Finnen was probably St. Finnian of Moville, who was born about 495 and died in 589. He was a pupil of St. Colman of Dromore, who was a noted teacher. "Mocholmog" was a kind of nickname for this St. Colman. It means "my dear little Column." (Catholic Online Saints). The Annals of Ulster also mention the death of Cellach Ua Cleircein, stating that he "fell asleep in peace," which may indicate that he was a peceable man and probably well thought of.

Both Annals mention the death of Glun Iarian Ui Cleirchen in 1045. It seems probable that he was from the northern Cleirchen family. He may possibly have been a brother of Cellach Ua Cleircinn who died in 1043.

In 1053, the Annals of Ulster report that Cu Macha MacCleirchen, steward of the Dal Cais was killed in a raid by the "men of Mag Itha."

For the year 1089, in the description of a battle, Druim-Ui-Clerichein is mentioned. A note in Keating's history of Ireland says : "DRUIM-UI-CLEIRCHEIN: i.e. O'Cleirchein's Ridge or Long Hill. In the Dublin copy of the ANNALS OF INNISFALLEN and the CHRONICON Scotorum, it is called Drumain-Ui-Cleirchein, which is the form of the name still retained. It is now anglicised Drummin and is the name of a townland and parish, in the arony of osma, and County of Limerick, about three miles north of Kilmallock."

In 1108, Oenghus Ua Clercein, "Patrick's steward in Munster" died, according to the Annals of the Kingdom of Ireland. Again, we don't know which Cleirchen family he belonged to, but it seems probably that he was of the Ui Fidgheinte family

According to the History of Ireland by the Four Masters, Maelcallan MacClerken was the son of Adam MacClerken and was Bishop of Clonfert in Galway in 1186. But according to the Annals of Loch Ce AD 1014-1590: Maelcalainn O'Cleirchen bishop of Glenn-da-Loch (in Wicklow) in Christo quievit (died) in 1186.