"Notre Dame's nickname is inherited from Irish immigrant soldiers who fought in the Civil War with the Union's Irish Brigade, (consisting of the 69th NY, 63rd NY, 88th NY, 116th Pennsylvania, and 28th Mass. Infantry Regiments)
Notre Dame's claim to the nickname is justified since its third president was a famous Irish Brigade chaplain whose ministrations at Gettysburg are commemorated in the "Absolution Under Fire," part of Notre Dame's permanent art collection. There are two identical monuments dedicated to him, one is on the battlefield at the Gettysburg National Military Park, and the other is on the campus of Notre Dame"
At Notre Dame it became more common after 1899 (much earlier than the "modern era") with Notre Dame leading Northwestern 5-0 at halftime of a game in Evanston, Illinois. The Wildcat fans (Ironic) began to chant, "Kill the Fighting Irish, kill the Fighting Irish," as the second half opened... The rest is history.
Common mistake from you folks down there, but Notre Dame used this far before Cathedral.