Callahan Family Name. An Irish surname, Callahan is from the Gaeic term ‘ceallach’ which means rivalry and strife. Another form ‘ceall’ means someone smart, with a bright head. Variations in spelling for Callahan include Ceilahan, Callaghan, Callihan, Callachan, Kallahan, Calahan, O’Callahan, O;Kallaghan, Kelagan and Callighan.