Career
Ó Cionnaith joined the Irish republican movement as a teenager, and in the late 1950s he became an activist with Na Fianna Éireann, the movement"s youth section. He strongly opposed the emergence of the Provisional Irish Republican Army, regarding its campaign as sectarian. During the 1960s and early 1970s, Ó Cionnaith developed a number of campaigning organisations including the Dublin Housing Action Committee, the Gaeltacht Civil Rights Movement (Coiste Cearta Síbialta na Gaeilge), the Resources Protection Campaign and the campaign to end the control by private landlords over the fishing rights to Irish rivers and lakes.
In the 1970s he became joint General Secretary of Official Sinn Féin (later Sinn Féin the Workers Party and ultimately The Workers" Party) along with Mairín De Burca.
Ó Cionnaith served as Director of International Affairs of the Workers" Party for years, and was the party"s representative in the United States during the early 1970s. He remained with the Workers" party after the split which led to the formation of Democratic Left.
He lost his Council seat in 1999 and was unsuccessful in standing for the 2002 Dáil election in Dublin North–West. On 15 February 2003 Seán Ó Cionnaith joined over 100,000 Irish people who participated in a major march in Dublin against the impending United States / United Kingdom led invasion of Iraq.
He died suddenly early the following morning.