The Irish surname Callahan  is derived from the Gaelic name Ó Ceallacháin meaning “descendant of Ceallachán,” a diminutive of the personal name Ceallach, meaning “bright-headed, or blonde haired,”  from cen meaning “head’ and lach meaning “light.” This name was borne by a 10th-century king of Munster, from whom many present-day bearers of the surname claim descent. County Cork is where most bearers of the name trace their ancestry to.