Ó Ceallacháin signifies descent from Cellachán Cashel. The personal name Cellachán consists of two parts, Cellach or Ceallach and the diminutive suffix -án. Donnchadh Ó Corráin argued that the word derives from cenn, “a head,” and loch, “shining, resplendent, bright.” Cendlach (shining head), followed by the diminutive suffix -án (genitive -áin), gives the form Cendlachán or Cellachán. The female form of the name is Ní Cheallacháin, that is, the daughter of Ceallachán.
Ancient Territory: The O’Callaghans established a lordship in County Cork along both banks of the Blackwater River between Kanturk and Mallow in the barony of Duhallow. Steady encroachment by English adventurers and speculators, however, imposed severe pressure on the Gaelic way of life. As a consequence of the rebellion of 1641 and the subsequent conquest by Oliver Cromwell, O’Callaghan lands were confiscated and the chieftain and his family were transplanted to County Clare.