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. In England the Callahan families live in the city of London and surrounding counties and to the north in Yorkshire and Lancashire. Some are also in Devon and Dorset to the southwest. In Ireland the region of Munster has high number with the Callahan name. In Scotland the Callahan population is centered in Lanarkshire and Stirlingshire. Within the United States the Callahan families are mostly in Massachusetts, New York, New Jersey, Ohio, Illinois, California and Pennsylvania. Famous: Harry M. Callahan (photographer), Americus F. Callahan (inventor of the window envelope in 1902), David Callahan (journalist), Ryan Callahan (professional ice hockey player), Bill Callahan (singer, musical and songwriter), John Callahan (actor) and Gene Callahan (movie film and television art and set director).