The kid on the skateboard who appears outside Robert Arkins' (Jimmy's) window during the first third of the movie when the band are recruiting members is the (now grown up) boy from the covers of U2's "Boy" (1980) and "War" (1983) albums. At the time The Commitments (1991) was filmed, he owned a skate shop in Dublin and was a champion skateboarder.
The Corrs, the highly successful Irish band, got their start by auditioning for the film, and they each won small roles. Andrea Corr (lead vocals & tin whistle) plays Jimmy's little sister Sharon. Jim Corr (guitar) is part of the Avant-Garde-A-Clue Band. Caroline Corr (drums) appears in the audience during the performance of "I Never Loved A Man". Finally, Sharon Corr (violin) can be seen playing violin with the country & western band that Bernie joins at the end of the film. John Hughes, the film's musical coordinator, became the band's manager. Director Alan Parker later asked Andrea for a role in the film/musical Evita (1996).
Jimmy is asked if he has brought Mississippi Burning (1988) (also directed by Parker) with him on the train. He is also asked if he has any "Hothouse Flowers", a band in which Maria Doyle Kennedy (Natalie) once sang backup for.
Glen Hansard had a difficult time during filming. A clash early on with director Alan Parker made it very difficult for him to enjoy the experience. It took him almost 20 years before he watched the full film.
Jimmy is waiting out front Gallagher's club (waiting for Wilson Pickett). Spray painted graffiti on the wall behind him is "killing joke". Although this may be familiar to UK viewers, it's not so well known in the US - Killing Joke is a UK-based punk band, who were popular in the LA punk scene in the mid-1980's.
When the boys are discussing a possible name for their band, one suggestion put forward - "The Liffey Lads" - was an in-joke nod to the film's screenwriters Dick Clement and Ian La Frenais who had co-written the earlier British TV sitcom The Likely Lads (1964).
The pool hall in this movie was a place called Ricardo's (now called Camden Deluxe) on Camden Street south of Temple Bar. It is now a bar and the upstairs junk room where the band rehearse is Poolhall. The scene outside the hotel where Wilson Pickett was supposedly staying was filmed outside the Mansion House, which is the official residence of the Lord Mayor of Dublin.
For their St Patrick's Day 2016 evening showing, Ireland's TV3 broadcast a version with the MGM logo, instead of the original Distributor 20th Century Fox, making the blatant 20th Century Fox's CBS-Fox Video product placement, in the Video Store really obvious. Additionally, instead of a Widescreen HD version, as on the original cinema release, this same broadcast used a 3:4 academy ratio cropped version, that was shown, with vertical sidebars, as would have been seen cropped on earlier TV broadcasts and the original VHS Home Video version. Presumably a widescreen HD version was unavailable.
Derek Scully, the bassist, is nearly electrocuted onstage from touching an electrified microphone. Later, at the hospital, Derek asks Jimmy, "Isn't this how the guy from AC/DC died?" Jimmy replies, "No, he choked on his own vomit." In 1980, Bon Scott, lead singer of AC/DC, reportedly died by choking on his own vomit while sleeping in his car outside a London house. However, in 1976, Keith Relf, guitarist of the Yardbirds, died from electrocution while playing an improperly-grounded guitar in the basement of his London home.
The trivia items below may give away important plot points.
In the epilogue to the film you can see Bernie singing in a band called The Brassers (this name is on the drums). In the book's closing chapters this is the band that Jimmy forms with Derek and Outspan after The Commitments have split up.
There's an unconfirmed fan theory that the unnamed character "Guy" in "Once", may actually be the same character as "Outspan Foster" that Glen Hansard earlier played in Alan Parker's version of "The Commitments". This plausible theory is based upon Glen Hansard's last scene in The Commitments (1991) where he is seen busking in the streets of Dublin. In the opening scene of Once he is also seen busking in the streets of Dublin. Using movie logic, it can legitimately be implied it is the same character, as its a massive coincidence otherwise.