The novel tells the story of the formation, rise, and fall of the Commitments, a soul band based in Dublin, Ireland.

The Commitments starts out with Outspan and Derek as band members and Jimmy as manager. Outspan and Derek want Ray in because he owns a synth; however, Jimmy does not like Ray much and insists that he should not be included in the group. Jimmy then recruits Declan and James and puts up an advertisement for additional band members in the Hot Press classifieds. Through this advertisement, Jimmy recruits three band members: Billy, Dean, and Joey the Lips. The full band then comprises Jimmy Rabbitte (manager), Outspan Foster (guitar), Declan Cuffe (vocals), Derek Scully (bass), James Clifford (piano), Billy Mooney (drums), Dean Fay (sax), Joey the Lips, and three backup vocalists: Imelda Quirk, Natalie Murphy, and Bernie McLoughlin. With Jimmy’s advice, the group does Dublin Soul with a focus on sex and politics and hopes to take Ireland by storm.

At the beginning, Jimmy has all the band members working to improve on their skills before their initial rehearsal. They hold meetings twice weekly in Joey the Lips’s mother’s garage and report on their progress. Jimmy gives them stage names to add flair and personality to their images: James the Soul Surgeon Clifford, Derek the Meatman Scully, Declan Blanketman Cuffe, Billy the Animal Mooney, Dean Good Times Fay, L. Terrence Foster, and Sonya, Sofia, and Tanya—the "Commitmentettes." Joey the Lips is the oldest member of the group and has experience in the music industry; he helps Jimmy to manage most of the group's rehearsals and mentors the band members on everything from dressing to appropriate stage behavior.

The group has its first performance at the Community Centre. Weeks later, they get a one-night gig at the Regency Rooms. The band becomes better with every live performance and receive mentions in the Northside News and the Herald; the publicity works to their advantage, earning them more long-term gigs in the surrounding environs. However, just as things are picking up, Billy leaves the group, citing irreconcilable differences between him and Declan. Declan's egotistical personality has isolated him from most of the group members, who would probably prefer to have him leave had it not been for his perfect voice. Mickah, who until then is the band’s bouncer at their live performances, steps in on the drums.

Many successful performances later, the band splits after a row over Imelda, whom many of the male band members secretly want. Imelda is seen kissing Joey, and all hell breaks loose. As Jimmy puts it earlier on in the story,

Imelda was holding the band together. Derek fancied her, and Outspan fancied her. Declan fancied her. He was sure James fancied her. Dean fancied her. He fancied her. Imelda had soul.

The story ends with Jimmy, Outspan, Derek, and Mickah toying with the idea of starting a new band with a focus on country punk, just them and the girls.