The Irish form of the name is Ó Ceallacháin and signifies descent from Cellachán Cashel mentioned above. 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.
In the course of the nineteenth century the O was abandoned and most people were identified simply as Callaghan or Callahan. As a consequence of the Gaelic revival in the early twentieth century people took up the O again and so Callaghans became O’Callaghans once more. The apostrophe is perhaps a distant reflection of the acute accent on the Irish Ó.
The O’Callaghans (from Ó Céileacháin in Irish) of the northern Counties of Louth, Armagh, and Monaghan have no connection with the Cork family and will not be considered here.