This ancient and honourable surname is of Irish and royal origins. Originally spelt as O'Ceallachan, it was composed of the elements O' meaning descendant of, and Ceallachan, a diminutive of the personal name Ceallach, meaning 'strife'. As such the 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, County Cork, after the Anglo-Norman invasion of 1170, they acquired large holdings near the town of Mallow, also in County Cork, and retained them until again dispossessed under the Cromwellian regime of 1651 - 1660. At this time the leading family of the clan were transplanted to County Clare, which remains their home. There have been a number of distinguished holders of the name including Father Richard Callaghan (1738 - 1807), the Jesuit educationalist; and two historians in the persons of Edmund O' Callaghan (1797 - 1883) and John Cornelius O' Callaghan (1803 - 1883); as well as the famous Victorian engineer Sir Francis O' Callaghan (1839 - 1909). In the United Kingdom James Callaghan, born in Wales, was Prime Minster and leader of the Labour Party in 1976 - 1979. The coat of arms most associated with the surname depicts a green mount on a white field, and on the sinister a hurst of oak trees with a wolf passant toward the dexter. The first recorded spelling of the family name is shown to be that of John O' Callaghan which was dated 1605, in the birth records of County Cork. Throughout the centuries surnames in every country have continued to "develop", often leading to astonishing variants of the original spelling.