Callahan is an Irish surname from Ó Ceallacháin, descendant of Ceallachán, a pet form of the old name Ceallach whose meaning is uncertain but is usually given as "bright-headed". It is one of the longest Irish surnames to become a first name in the United States, and it shortens naturally to Cal. Callahan entered the US top 1000 in 2019 and has climbed every year since 2015, from number 1806 then to number 227 in 2025, its best rank to date.