McIlroy was born in Belfast and moved to English club Manchester United in 1969 becoming Matt Busby's final signing. He made his debut on 6 November 1971 in the Manchester derby against Manchester City scoring in a 3–3 draw. He drifted in and out of the side and played in 31 matches in 1973–74 as Manchester United suffered a rare relegation. He was an ever-present in 1974–75 playing in all of the club's 51 fixtures as they gained an instant return to the First Division. On their return they finished in third place and also reached the 1976 FA Cup Final where they lost 1–0 to Southampton.