The thing is that these Irish nicknames were originally bestowed by Irish immigrants upon themselves.
While various versions of the origin of Notre Dame’s nickname exist, they all draw on links between the university and Irishmen. Most of the missionary priests and brothers who founded the school were from Ireland, influential early president William Corby had been a chaplain with the original “Fighting Irish” (the Union Army’s Irish Brigade in the Civil War), Irish revolutionary Éamon de Valera was enthusiastically welcomed to the school during his barnstorming tour of the US in 1919, and during the 1910s and 20s (the Knute Rockne era, while the team as yet had no formal nickname), the press still often referred to their football team as the Papists or the Dirty Irish, so Rockne in his role as natural salesman co-opted the name and turned it into the Fighting Irish. Even today, the University of Notre Dame proudly boasts that its Keough-Naughton Institute for Irish Studies is the largest center for the study of the Irish language outside of Dublin.
Meanwhile, the Boston Celtics were named after the old New York Celtics, who started as a settlement house team in 1914, made up mostly of Irish immigrants in Hell’s Kitchen. Internationally, perhaps the most famous “Celtic” sports team is Celtic Football Club in Glasgow, Scotland, founded by Irish immigrants and historically associated with the Irish diaspora in Scotland and Irish nationalism. (Even with the past ten years, the team has been fined at least twice by the Union of European Football Associations because the fans were chanting pro-IRA slogans and displaying “illicit banners” commemorating IRA hunger striker Bobby Sands.)
I am unaware of any similar link between the Cleveland team’s name and any Native American tribe.
