Career
In 1957 O'Brien captained the team to the All-Ireland title. At club level O'Brien was a three-time county club championship medalist with St. Mary's. A long-time amateur musician, in 1962 O'Brien became a professional musician when his band the Clubmen went professional.
O'Brien played the piano accordion for the band rather than the more typical Irish button accordion. Dermot O'Brien and the Clubmen had considerable musical success, with their hit single "The Merry Ploughboy" (a cover of an old song about joining the Irish Republican Army) reaching the top of the Irish Singles Chart in only seven days and holding that position for six weeks in late 1966. The single was rushed to market to compete with a Top 100-ranking cover of the same song by the Abbey Tavern Singers.
In the late 1960s, O'Brien began his own RTE show called "The Styles of O'Brien".