The plot follows an Irishman and his daughter, who steal a leprechaun's magic pot of gold and emigrate to the American South, where they become involved in a dispute between rural landowners and a greedy, racist U.S. senator.

A scheming immigrant named Finian McLonergan arrives in America from his native Ireland, having absconded with a crock full of gold secreted in a carpetbag, plus his daughter Sharon in tow.

His destination is Rainbow Valley in the fictional state of Missitucky, where he plans to bury his treasure in the mistaken belief that, given its proximity to Fort Knox, it will multiply.

Complications arise when Rawkins, believing that there is gold in Rainbow Valley, attempts to seize the land from the people who live there, and makes some racial slurs while doing so.

Og meets with Susan on the bridge under which she has hidden the gold and, as he develops romantic feelings for her in absence of Sharon, wishes that she could talk.

In 1948, Metro-Goldwyn-Mayer wanted to acquire it as a vehicle for Mickey Rooney; however, MGM balked at Harburg's price of $1 million for the rights and complete creative control.

[5] At that time, Harburg stated that he was told that part of the reason it was so difficult to get a film version made of Finian's Rainbow was because Hollywood was wary of making fantasy musicals.

[8][full citation needed] In September 1966, Warner Bros. announced that it had the rights, and would make a film produced by Joseph Landon and starring Fred Astaire, with the aim to get Tommy Steele as the leprechaun.

[12] With the then-unreleased Camelot having proven to be more costly than anticipated, and its commercial prospects still unknown, Jack L. Warner was having second thoughts about undertaking another musical film, but when he saw Petula Clark perform on her opening night at the Cocoanut Grove in the Ambassador Hotel in Los Angeles, he knew that he had found the ideal Sharon.

[13] Warner decided to forge ahead and hope for the best, despite his misgivings of having nearly-novice "hippie" director, Francis Ford Coppola, at the helm.

While a construction crew transformed more than nine acres of the Warner Bros. back-lot into Rainbow Valley, complete with a narrow gauge railway, schoolhouse, general store, post office, residential houses and barns, Coppola spent five weeks rehearsing the cast.

[15] In the liner notes that she wrote for the 2004 Rhino Records limited, numbered-edition CD reissue of the soundtrack album, Clark recalls that Golden Age-Hollywood Astaire was puzzled by Coppola's contemporary methods of filmmaking, and balked at dancing in "a real field with cow dung and rabbit holes".

[16] Finian's Rainbow proved to be Astaire's last major movie musical, although he danced with Gene Kelly during the linking sections of MGM's That's Entertainment, Part 2 (1976).

[17] Because preview audiences found the film overly long, the musical number, "Necessity", was cut before its release, although the song remains on the soundtrack album.

In August 2012, Petula Clark told the BBC Radio 4 show, The Reunion, that she and her fellow cast members smoked marijuana during the filming of the movie.

[21] Finian's Rainbow was released in major cities as a roadshow presentation, complete with intermission, at a time when the popularity of movie musicals was on the wane.

The film was dismissed as inconsequential by many critics who were startled by Astaire's aged appearance and found Steele's manic performance as Og, the Leprechaun, annoying.

In The New York Times, Renata Adler described it as a "cheesy, joyless thing", and added, "There is something awfully depressing about seeing Finian's Rainbow... with Fred Astaire looking ancient, far beyond his years, collapsed and red-eyed... it is not just that the musical is dated... it is that it has been done listlessly and even tastelessly.

John Mahoney of The Hollywood Reporter wrote that Clark "invites no comparisons, bringing to her interpretation of Sharon her own distinctive freshness and form of delivery".

[15] Joseph Morgenstern of Newsweek wrote that Clark "looks lovely" and "sings beautifully, with an occasional startling reference to the phrasing and timbre of Ella Logan's original performance".