The name Callahan originates from the Irish Gaelic language and culture, specifically from the word 'O'Callaghan', which means 'descendant of Callaghan'. The name Callaghan is derived from the Irish words 'calla' meaning 'battle' and 'ghan' meaning 'valor' or 'green branch'. Thus, the name Callahan essentially means 'brave in battle' or 'descendant of the green branch'.

The name has its roots in ancient Ireland, and it is believed to have first appeared in the 10th century AD. The Callahans were a prominent clan in County Cork, Ireland, and they were known for their valor and military prowess. The name was widely used by members of this clan and their descendants.

One of the earliest recorded instances of the name Callahan can be found in the Annals of Inisfallen, which is a chronicle of medieval Irish history. It mentions a chieftain named Callahan mac Cinneidigh, who lived in the 11th century.

Throughout history, there have been several notable individuals who bore the name Callahan. One of the most famous was Daniel Callahan (1904-1986), an American philosopher and ethicist who made significant contributions to the field of bioethics. Another notable Callahan was John Callahan (1951-2010), an American cartoonist known for his dark and irreverent humor.

Other notable individuals with the name Callahan include John Callahan (1833-1912), an American Civil War veteran and recipient of the Medal of Honor; Michael Callahan (1945-2016), an American basketball player who played in the NBA; and Connie Callahan (1929-2020), an American singer and actress who performed on Broadway.

The name Callahan has also been used in literature and popular culture. For example, in the novel 'The Big Sleep' by Raymond Chandler, one of the main characters is named Eddie Callahan. Additionally, in the television series 'Scrubs', one of the recurring characters is named Callahan.