Most of this film was shot in Eckley, Pennsylvania. Paramount Pictures saved the town from being destroyed--it was slated to be demolished for strip mining--but after the movie was filmed, the town's land was donated to the Pennsylvania Historical and Museum Commission. The town is now a museum. Several structures built for the movie still survive.
The film was a notorious flop when it was released in 1970, earning back a little over ten percent of its 11 million dollar budget, an enormous sum for the time (approximately 55.4 million in 2005 dollars). The film's failure solidified Sean Connery's reputation for being box-office poison outside of James Bond and torpedoed co-star Richard Harris' chance at superstar status. Connery's career continued its non-Bond stall throughout the 1970s (with the notable exception of John Huston's The Man Who Would Be King (1975)), and Harris' career as a leading man continued to decline until it went into eclipse in the middle of the same decade. While Connery would re-establish himself as a superstar in the 1980s, Harris never again was a front-rank star, though he wound up as a respectable and in-demand character actor later in his career.
During filming, Sean Connery and Richard Harris became good friends. Harris later agreed to play the cameo role of King Richard the Lionheart in Robin and Marian (1976) as a favor to Connery, and in the late 1980s, there was talk of them making another film together, but this never happened.
According to Cinematographer James Wong Howe, Director Martin Ritt wanted to shoot this film in black-and-white, but was not allowed to do so by Paramount. By 1970, studios were concerned that black-and-white films would not make as much money when broadcast on television, possibly for fear that they would be considered "old" films.
Richard Harris received top first billed, Sean Connery received second billing. Reportedly, Connery did not mind being billed in second place, once pointing out to an interviewer, "For the amount of money they're paying me for this, they can put a mule ahead of me". Similarly, Connery also once said, "For one million dollars, they can bill me after the janitor!". The billing order was of note for this film because Connery was a big star who had played James Bond whilst Harris had not ever achieved super-star status. Top billing usually goes to the bigger star and based on this usual criteria, Connery would have been top first billed.
According to Shane R. Burridge's IMDb review for the film, the actual real-life Molly Maguire person "...was a peasant girl in 17th century Ireland, who supposedly led an uprising against rent collectors". Hence, the connection with the group and title "The Molly Maguires" as the miners rebel and dress up as women.
The film's two leads have both played King Arthur on screen, Sean Connery in First Knight (1995), and Richard Harris in Camelot (1967), as well as on television, in a videotaped presentation of the 1980 Broadway revival of the musical (See: Camelot (1982) (TV)).
The film's title refers to a secret nineteenth century Pennsylvania group of Irish immigrant coal miners, who fought oppressive mine-owners, in order to improve their low wages and poor working conditions. They did this by using sabotage and terrorist tactics.