This ancient and honourable Irish name is an Anglicized form of the Gaelic "O' Ceallachain" composed of the elements "O" meaning "descendant of" plus "Ceallachan", a diminutive of the personal name "Ceallach" meaning contention or strife. This name was borne by a 10th Century King of Munster, from whom many present day bearers of the surname claim descent. 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 were transplanted then to Co. Clare. There have been a number of distinguished Irishmen of the name; among them Father Richard Callaghan (1738 - 1807), the Jesuit educationalist; two historians in the persons of Edmund O' Callaghan (1797 - 1883) and John Cornelius O' Callaghan (1803 - 1883); and Sir Francis O' Callaghan (1839 - 1909), the engineer. The family Coat of Arms depicts a green mount in base and on the sinister side a hurst of oak trees issuant therefrom a wolf passant toward the dexter all proper, on a silver shield. The first recorded spelling of the family name is shown to be that of John O' Callaghan which was dated 1605, Birth Records of Co. Cork, during the reign of King James 1st of England, 1603 - 1625. Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.