O'Brien began his career with Shelbourne before transferring to Manchester United.
He was then transferred to Notts County in 1974[1] for £45,000 without making the first team at Old Trafford.
At Meadow Lane he created a club record in 1979–80 when he became the first full back to finish as leading scorer. He spent ten years at County making over 300 Football League appearances.
He also won five caps for the Republic of Ireland, making his debut on 24 March 1976 in a 3–0 home win over Norway in a friendly game.[2]