There are several legends of how Notre Dame came to be the "Fighting Irish."
One story suggests the moniker was born in 1899 with Notre Dame leading the Northwestern Wildcats 5-0 at halftime of a game in Evanston, Illinois. The Wildcat fans began to chant, "Kill the Fighting Irish, kill the Fighting Irish," as the second half opened.
Another tale has the nickname originating at halftime of the Notre Dame-Michigan game in 1909. With his team trailing, one Notre Dame player yelled to his teammates — who had names like Dolan, Kelly, Donnelly, Glynn, Duffy and Ryan - "What's the matter with you guys? You're all Irish and you're not fighting worth a lick." Notre Dame came back to win the game and the press, after overhearing the remark, reported the game as a victory for the "Fighting Irish."
Another possible origin is the violent 1924 confrontation between Notre Dame Students and faculty and the anti-Catholic Ku Klux Klan in South Bend. This event is described in Todd Tucker's book Notre Dame Vs. the Klan: How the Fighting Irish Defeated the Ku Klux Klan.
The most generally accepted explanation is that the press coined the nickname as a characterization of Notre Dame teams in the 1920's as a result of preexisting Irish stereotypes, the widely reported events of 1924 (although after their 10–7 loss to Iowa, Notre Dame was referred to as the "Irish" in a newspaper article about the game), and the grit, determination, and tenacity of Coach Knute Rockne's football teams of the era.
Although Notre Dame alumnus Francis Wallace popularized the term "Fighting Irish" in his New York Daily News columns in the 1920s with respect to the university, as early as the Civil War Father Corby and the Irish Brigade of the Union Army had been dubbed "The Fighting Irish."