Arthur, William (1857) An Etymological Dictionary of Family and Christian Names. New York: Sheldon, Blakeman. Public Domain.
GALLAGHER
As described in historical dictionaries
An Etymological Dictionary of Family and Christian Names (1857)
(origin: Gaelic.) From Gallach, valiant, brave, and er put for fear, a man. Air is a common termination of nouns, and changes into eir, ir, or, oir, and uir, its etymon being fear, a man.
Origin: Celtic
Region of origin: British Isles
Country of origin: Ireland
Religion of origin: Christian
Language of origin: Gaelic