Callahan, the phonetic spelling of Callaghan, is a rhythmic jig of a name whose history harks back to the ancient King of Munster. In the Dirty Harry movies, the ClintEastwood character is Harry Callahan.
Spider Robinson wrote a series of books about a bar called Callahan's, run by Mike Callahan.
A teacher/lawyer in Legally Blonde Alexander Callahan, known as "Cal" is a character in the Hex Hall series by Rachel Hawkins.
Dirty Harry aka Harry Callahan