Together with Miah Dennehy, John Herrick, Gerry Coyne and Dinny Allen, he was a prominent member of a successful Hibernians team managed by Dave Bacuzzi.
[13][12] O'Mahony also served as team captain [14] and played for Hibernians in Europe, including in a 1970–71 Inter-Cities Fairs Cup tie against Valencia.
[16][17][18] The highlight of his time with Alberts was guiding the club to the 1977–78 League of Ireland Cup final which they lost to Dundalk.
In the first round they held Bayern Munich to a 1–1 draw at Musgrave Park before eventually losing the tie 3–1 on aggregate.
In 1992–93, with a team that included Phil Harrington, John Caulfield, Pat Morley, Paul Bannon, Gerry McCabe and Dave Barry, O'Mahony guided Cork City to their first Premier Division title after a series of three-way play-offs that also involved Bohemians and Shelbourne.
Two weeks prior to his death, he had attended a 20-year reunion of Cork City's 1992–93 Premier Division winning team.
An obituary in the Irish Examiner remarked that O'Mahony had achieved success at three of Cork city's sports stadiums.
As a manager of Cork City, he oversaw a draw with Bayern Munich at Musgrave Park and then won the Premier Division at Turners Cross.