"Irreplaceable" is a mid-tempo ballad with pop and R&B influences. Written in the key of B major, the beat is set in common time and moves at a moderate 88 beats per minute. Beyoncé's vocal range in the song spans nearly two and a half octaves, from B2 to E5. While most of the songs on the album are aggressive and uptempo, her voice on "Irreplaceable" is toned down. "Irreplaceable" uses a gently strummed acoustic guitar, following the B5–F5–Cm7–E6/9 chord series. Hermansen and Eriksen combined the classic chord progression on an acoustic guitar, a modern-sounding 808 drum beat and cellos. Al Shipley of "Stylus Magazine" noted that the guitar strum can be found in Rihanna's 2007 single "Hate That I Love You", a song co-produced by Stargate and Ne-Yo. Rob Sheffield of "Rolling Stone" wrote that Chris Brown's 2007 single "With You", another Stargate-produced song, also features the same element. He wrote, "'With You' is the convincer, even if you can instantly tell that producer Stargate was just trying to roll out 'Irreplaceable' one more time." while "Billboard" magazine wrote that it "leans a bit too heavily" to the song.