Callihan is likely derived from an Irish surname form related to Callahan, originally meaning descendant of Ceallach.
Callihan is a variant of Callahan, an Irish surname of distinguished antiquity, derived from the Gaelic Ceallachán — a diminutive of Ceallach, meaning "bright-headed," "lover of churches," or possibly relating to an old word for strife. The Callaghan sept was historically centered in Munster, in County Cork, and the name appears prominently in medieval Irish annals. Murchad Mac Ceallacháin was a ninth-century king of Munster, and the name has carried dynastic associations ever since.
As surnames migrated to given names in the twentieth century — a trend accelerated in American naming culture — Callahan and its variants found new life as first names, particularly among families of Irish descent wishing to honor ancestry without using a common saint's name. The spelling Callihan softens the name slightly, shifting it from the blunt Irish-American masculine feel of Callahan toward something that reads as more gender-flexible, with the -han becoming -han, a small change that opens considerable perceptual space. In contemporary usage Callihan occupies the same fashionable territory as surnames-as-first-names like Sullivan, Brennan, and Finnegan — names that carry Celtic heritage with a modern edge.
Its four syllables give it a certain grandeur, and its nicknames are satisfying: Cal, Calli, Han. It suits an era in which parents want names that sound distinguished and old-world without being common, and Irish heritage remains one of the most beloved sources for that sensibility in English-speaking countries.