genealogies of the families
(O)CALLAGHAN, (Kelaghan) The name of O'Callaghan, in Irish O Ceallachain, was taken from Ceallachan (Callaghan), King of Munster (d. 952), the eponymous ancestor of the sept.
Dispossessed of their original territory in the barony of Kinelea, Co. Cork, after the Anglo-Norman invasion, they acquired a large area of north Co. Cork near Mallow and retained it until again dispossessed under the Cromwellian regime. The leading family of the sept was transplanted then to Co. Clare, where the village of O'Callaghan's Mills bears their name and where they are represented by the family of O'Callaghan (now O'Callaghan-Westropp) of Lismehane.
The humbler members of the sept, as was usually the case, were not transplanted; and to-day Co. Cork is the area in which O'Callaghans are chiefly to be found. Although they are mainly concentrated in that area the total number of O'Callaghans and Callaghans in Ireland to-day is about 13,000, which places the name among the forty most numerous in the country.
It may here be observed that Callaghan is one of those names in which the resumption of the prefix 'O', dropped during the period of Gaelic submergence, has been most widely resumed: fifty years ago Callaghans without the 'O' outnumbered those who used the prefix by five to one, while to-day O'Callaghans are much more numerous than Callaghans.
In addition to King Callaghan mentioned above there have been a number of distinguished Irishmen of the name, among them Father Richard Callaghan (17381807), the Jesuit educationalist, two historians in the persons of Edmund O'Callaghan (1797-1883), and John Cornelius O'Callaghan (1805-1883), and Sir Francis O'Callaghan (1839-1909), the engineer.
The name is also one of distinction in the records of the Irish Brigades in France. It is a curious fact that the Abbe John O'Callaghan (1605-1654), who was a very promi nent Jansenist in France, gives the name of his father, a gentleman of Macroom, Co. Cork, as MacCallaghan.
The records of the O'Callaghans in Spain are very extensive in the archives of that country. O'Callaghan is one of the few families of which a modem Chief of the Name is certified by the Genealogical Office. The present holder of that designation is a citizen of Spain.
The name O'Callaghan is sometimes found in Oriel (Armagh, Louth and Monaghan). This is an entirely different sept. O'Ceileachain in Irish, properly anglicized O'Kelaghan or Kealahan; this name has become O'Callaghan in some families through a not uncommon process of attraction, but the form Kelaghan is still in use in Co.Westmeath
Source: Irish Families by Edward MacLysaght MA, D Litt, MRIA - Irish Academic Press 1991