“It’s a Great Day for the Irish” is Judy Garland’s song. The song was specifically written for her, and she debuted it in 1940 in the film “Little Nellie Kelly.”
Although Garland had already made 16 films, including “The Wizard of Oz,” she was most often in supporting roles. “Little Nellie Kelly” was intended to test whether she could she could sustain a lead career. The film was based on George M. Cohan’s 1922 Broadway show and capitalized on Garland’s Irish-American identity. (She was 1/4 Irish, and had a maternal grandmother from Ireland.)
“Little Nellie Kelly” tells the story of two generations of romantic struggle, starting in Ireland, when the title character marries a man her father objects to and he promises never to speak to her husband again. She dies in America, and her daughter likewise grows up to fall in love with a man her father objects to. Both mother and daughter were played by Judy Garland.
“It’s a Great Day for the Irish” was written for a scene taking place in New York’s annual St. Patrick’s Day Parade on Fifth Avenue, during which Garland marches with her family and points out the various Irish-Americans also in the parade. As a result, the song has become a St. Patrick’s Day favorite.
The song was authored by Scots Irish composer Roger Edens, one of the great arrangers during Hollywood’s Golden Age. He was Judy Garland’s original vocal trainer and worked with her over her entire career.
The use of the phrase “great day for the Irish” is older than the song. It dates back to at least 1870, when The Leavenworth Bulletin of Leavenworth, KS, used the phrase in reference to St. Patrick’s Day that year. In fact, there were earlier songs with the same title, although there is little documentation about them.
The 1940 song has been extensively covered; according to Wikipedia, singers include Bing Crosby, Ruby Murray, Daniel O’Donnell, and The Clancy Brothers.