According to O'Donovan (1848) and The O'Callaghan Clan website, "it is from one of the Eoghanacht kings, Ceallachan (d.964), that the family trace their descent. Murchadg Ua Ceallachain, a grandson of this king who lived in the early eleventh century, was the first to transit the surname hereditarily. His nephew Carthach was the ancestor of the MacCarthys." Surnames were introduced into Ireland by Brian Boru during the early 11th century. Ceallachan probably died in 954 AD. His son Donnchad was also king of Munster for about 4 years years (there were three other kings between Ceallachan and Donnchad). Donnchad died in 963 AD. "Cellachan Caisil mac Buadachan" means the same as Ceallachan of Cashel, son of Buadachan. His son is sometimes referred to as Donnchad mac Cellachan, Donnchad Ceallachan, or Donnchad, son of Ceallachan.
Interestingly, there also was a later Saint Callaghan who was a monk at Clontibret (Coghlan et al., The Book of Irish Names, 1989:14) and a Callahan castle at Clonmeen that was destroyed by William of Orange. At one point in later history the Callahans "settled on the banks of the Blackwater, west of Mallow, where they became the chiefs of a territory called after them 'Pobul Ui Cheallachain' " (C. Thomas Cairney, 1989, Clans and Families of Ireland and Scotland, An Ethnography of the Gael, A.D. 500-1750; McFarland & Co. Jefferson NC; p.123). The Poems of Egan O'Rahilly (1911) have two poems written in the early 1700's about the death of Domhnall O'Callaghan in 1724 and describe his genealogy, life at Clonmeen Castle, and the Callahan coat of arms. The Callahan crest is based upon the image of a wolf emerging from an oak grove, i.e. "Ar. in base a mount vert on the dexter side a hurst of oak trees issuant therefrom a wolf passant towards the sinister all proper." (Cairney 1989:164). There are also extensive records in continental archives of Spanish Knights of Irish Origin including Matheo O Calaghan (1722), Knight of Santiago (M. Walsh 1960).
The Inauguration of Ceallachan of Cashel as the King of Munster c. 940/44 A.D.
"Each put his hands between those of the prince, the royal diadem was placed on his head; it was announced to the people that Ceallachan, the son of Buadhachan, the son of Lachtna, the son of Ardghal, the son of Sneadhghus, the son of Faolghus, the son of Natfraoich, the son of Colga, the son of Failbhe Flan, the son of Aodh Dubh, the son of Criomhthann, the son of Feidhlimidh, the son of Aonghus, the son of Conall Corc, the son of Lughaidh, the son of Oilill Flannbeg, the son of Fiacha Muilleathan, the son of Eoghan Mor, the son of Oilill Olum, was sovereign prince and ruler of Leath Mogha, and the royal shout proclaimed the public approbation" (Gleeson, Cashel of the Kings; cited in Roger Chatterton Newman's Brian Boru, King of Ireland, 1983 p.54).
[The names described above correlate fairly well with the geneaological table of the Kings of Munster found in Moody et al. (1984:136). According to Moody et al., the Aonghus referred to in the geneaology described above died c. 490/92 A.D., suggesting that this geneaology extended well back in time. Moody et al. would appear to contradict the unsupported statement of Newman that, "The quotation, given by Gleeson in Cashel of the Kings, is an interesting, if genealogically dubious, insight into the coronation of an Irish king"(1983:194)]. Newman's statement of skepticism about Ceallachan's ancestry is based on his view that historians should not necessarily take such things at face value and on an assumption that it would have been politically advantageous to have a lengthy pedigree going back to the beginnings of the Eoghanacta in the second century with Eoghan Mor and Cormac Cas, which it certainly would have been, but that does not mean it was not also true. Newman is the biographer of Brian Boru whose Dalcassian father, Cinneide (Kennedy), was Ceallachan's rival and under their system of tanistry, Ceallachan's successor for the Kingship of Munster. Cinneide was later killed during a battle between the Dalcassian's and Ceallachan's Eoghanacht. The Dal Cas (Dalcassians) were the traditional kings of Thomond in north Munster, and the Eoghanact were traditional kings in south Munster. The kingship of Munster was determined by an election and perhaps tanistry. In theory it may have alternated between two groups, but this has been questioned by some historians.
More genealogical information:
Callahan and MacCarthy Genealogy chart, 954 A.D.-1773 A.D. (O' Donovan 1841:64).
Detailed genealogical tables of the Kings of Munster (Eoganacht) to 1024, the succession list of the Kings of Munster to 1119, and the early MacCarthys of Desmond beginning with Carthach, King of Eoganacht Caisil (d. 1045), can found in A New History of Ireland Vol. IX Maps, genealogies, lists edited by T.W. Moody, F.X. Martin, and F.J. Byrne, 1984, Clarendon Press: Oxford pp.136,154-157, 203-206.
According to O'Donovan (1848:12) Callaghan of Cashel is also mentioned in a poem on the Triumphs of the Kinel-Owen composed by Flann of the Monastery, preserved in the Book of Glendalough (fol.147, b.a.) a MS. of the twelfth century in the Manuscript Library of Trinity College, Dublin.
Earlier Callahan ancestry is further referenced on the Callahan WWW Home Page (see the Links page). For example: "The year 489 is notable for the death of Aenghus, son of Nadfreach, King of Munster, the common ancestor of the MacCarthy's, O'Keefe's, O'Callaghan's and O'Sullivan's...."
For genealogical pedigrees before and after Ceallachan of Cashel check The Book of Munster at http://www2.smumn.edu/uasal/eoghan.html. The website indicates that The Book of Munster was "Written in 1703, [by] Rev. Eugene O'Keeffe, Parish priest and Poet of Doneraile, North Cork." As an example of what is at the website, here is part of the line including, but mostly BEFORE, Ceallachan. The Book of Munster starts earlier with Oilill Olum and the beginning of the Eoghanacht genealogy.
"(This is the stem of the MacCarthy and O'Callaghan Pedigree)
The Race of Failbhe Flann here:-
Failbhe Flann, died 637, son of Aodh Dubh son of Criom Thainn
Here is another line including, AFTER, Ceallachan of Cashel:
Murchadh son of Donnchadh son of Ceallachan Caiseil had one son
Cathaoir Modartha's family: Donnchadh, Tadhg, Ceallachan, and Conchubhar.
(There are some genealogies of minor branches of O'Callaghan)"
[source: The Book of Munster].
Ceallachan's pedigree is also given in Geoffrey Keating's History of Ireland as follows:
"105 Donnchadh, son of
Left: "Cashel of the Kings" on the Rock of Cashel, in Co. Tipperary is atop a limestone rock that rises 200 feet above the surrounding plain..
However, since the descendants of Failbhe excelled those of Fingin
in prowess and noblity of deeds they are made founders in the genealogy
here, son of
There is some additional post Ceallachan genealogy at http://www2.smumn.edu/munster/ocallaghan.html
Left: Thumbnail of Lismore Castle, (after a photo by Desmond Guinness and William Ryan, 1971 Irish houses and castles, Thames & Hudson; London, p. 281). In 1814, during repairs to the castle, workmen found a wooden box inside a walled up passage. Inside the wooden box was a crosier, now in the National Museum, Dublin, inscribed with the information that the crosier was made for the bishop of Lismore who died in 1113 and the Book of McCarthy Reagh, popularly known as the Book of Lismore. One of the sections of the book was the saga of Ceallachan, now known as Caithreim Ceallachan Caisil. Other parts of the book included stories about the Lives of Saints, the reign of Charlemagne, and the travels of Marco Polo. Other copies of the saga's prose and poems survived independently. On June 20, 1629 the Book of Lismore was in Timoleague Abbey in the possession of Michael Clery, one of the authors of the Annals of the Four Masters. According to Whitley Stokes (1890), "The Book of Lismore was compiled from the lost Book of Monasterboice and other manuscripts in the latter half of the fifteenth century, for Finghin mac Carthaigh Riabhach and his wife Catherine, daughter of Thomas, eighth earl of Desmond" (Stokes 1890:v).