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Callahan is an Irish surname and a boy's name meaning "bright-headed". It is derived from the Gaelic name O'Ceallachain, which means "descendant of Ceallachan". Ceallachan was a personal name that means "bright-headed" or "strife". Callahan is also the name of a 10th-century king of Munster from whom many present-day bearers of the surname are descended.