O'Neill joined Stella Maris at the age of twelve before moving onto Home Farm. He also played for an FAI Youth Selection against a Liverpool County FA XI at Goodison Park, and in 1958 he played against the youth teams of some of the leading European clubs in a tournament in Germany.
O'Neill signed for Arsenal as an eighteen-year-old, initially as an amateur but subsequently turned professional in April 1959. He spent the next two seasons playing for Arsenal Reserves, making just 2 appearances in the English League. On both occasions, during the 1960–61 season, he deputised for the injured Danny Clapton. He made his senior debut for Arsenal on New Year's Eve 1960 in a 5–3 away win against Nottingham Forest and then played again in a home game against Blackpool.
Shamrock Rovers
O'Neill signed for Shamrock Rovers for £3000 in September 1961 after playing well on Rovers' tour of North America that summer. He scored twice on his debut against Waterford United on 17 September, in a 4–0 win in the League of Ireland Shield. He went on to become a prominent member of the Rovers team that won the FAI Cup six times in a row during the 1960s. He, along with Pat Courtney, is a holder of the six in a row medals. During his career with Rovers he played over 300 games. His teammates at the club during this era included Liam Tuohy, Johnny Fullam, Pat Dunne, Bobby Gilbert, Mick Leech and Paddy Mulligan. During the 1965–66 season O'Neill scored 6 goals during the FAI Cup run including one in the final against Limerick. In 1967, he also scored a penalty in the final, a 3–2 win against St. Patrick's Athletic. He scored a further 2 goals during the 1968–69 Cup run. During the summer of 1967, O'Neill also played for Rovers when they competed as Boston Rovers in the United Soccer Association league. O'Neill also scored 2 goals for Rovers, one in each game, during a European Cup Winners Cup tie against CA Spora Luxembourg, helping them to an 8–2 aggregate win . In total he played 18 times in European competition.
Between 1961 and 1971, while playing for Shamrock Rovers, O'Neill made 20 appearances and scored 1 goal for the Republic of Ireland. He holds the record for the number of international caps won by a League of Ireland player. He made his international debut on 8 October 1961in a World Cup qualifier against Czechoslovakia at Dalymount Park. O'Neill finished on the losing side as the Republic lost 3–1. He scored his only goal for the Republic on 11 November 1966 in a 2–1 home win against Turkey in a European qualifier. He made his last appearance for the Republic on 10 October 1971 in another European qualifier away to Austria. On that day the Republic lost 6–0.
Managerial career
He served as player-manager for Rovers after being appointed to the position on 19 October 1969. He guided the club to the league title play-off at the end of the 1970–71 season. O'Neill was later assistant manager of Dundalk between 1978 and 1979, winning the League of Ireland and FAI Cup double.
