[1] O'Brien made his first appearance for the Louth team during the 1952 championship and was a regular member of the starting fifteen until a broke finger ended his career in 1960.

[3] 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 a Jeremiah Lynch/Dominic Behan 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.

[5] In 2018, Brown University students started a Dermot O'Brien appreciation club.