O'Callaghan, along with its variants (O')Callagan, Callahan etc., comes from the Irish Ó Ceallacháin, from the personal name Ceallachán, a diminutive of ceallach. This was traditionally taken to mean "frequenter of churches", but is now thought to be a much older word meaning "bright-headed". The personal name was much in favour among the Eoghanacht, the tribal grouping who controlled the kingship of Munster before the rise of Brian Ború of the Dál gCais, and it is from one of the Eoghanacht kings, Ceallachán(d.954 A.D.) that the family trace their descent. Murchadh Ua Ceallacháin, 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, and a bloody succession feud between the MacCarthys and the O'Callaghans continued well into the twelfth century, ending with the MacCarthys in the ascendant.
By the end of the thirteenth century, the O'Callaghans had taken decisive possession of that part of Co. Cork which came to be known as Pobal Ui Ceallacháin, O'Callaghan's Country. This was a very large area on both sides of the river Blackwater west of the modern town of Mallow. Here their principal bases were the castles at Clonmeen and Dromaneen, and from them they retained virtually uninterrupted control for over four centuries, continuing many of the earlier Gaelic customs. The most notorious of these was the creach or cattle-raid; one Donncha, chief of the family from 1537 until his undeservedly peaceful death in 1578, was reputed to have carried out two hundred raids in every county of Munster, evidently regarding the creach as a vital part of his cultural inheritance.
Eventually, inevitably, the family suffered great losses in the wars of the seventeenth century; in 1643 no fewer than seventeen O'Callaghan "gentlemen" were declared outlaws, and in the confiscations following the final surrender of 1652, they lost 24,000 acres, 20,000 of which was deemed to have been the property of the ruling chief, Donncha O'Callaghan. He and his extended family were transplanted to east Clare, where they obtained land in the barony of Tulla. The village of O'Callaghan's Mills records their continued presence.
Despite the transplantation, of course, the vast majority of O'Callaghans remained in their ancestral homeland, most as tenants of the newly-installed English owners, some few having regained possession of small parts of their old lands. Clonmeen castle was ruined in the wars of the seventeenth century, but members of the family remained in possession of the lands around it until the mid eighteenth century. The Hibernian Chronicle of March 6th 1778 records the death of Robert O'Callaghan, "the last of the ancient family of O'Callaghan of Clonmeen".
Like so many others from the old Gaelic aristocracy, members of this Clare family emigrated to continental Europe. Cornelius O'Callaghan entered the army of Spain in 1717. In 1944 one of his descendants, Don Juan O'Callaghan of Tortosa was recognised by The Genealogical Office as the senior descendant in the male line of the last inaugurated Chief, the Donncha who was transplanted to Clare.