He was a member of the Irish Chile Solidarity Committee and attended the 1st Party Congress of the Cuban Communist Party in 1984. He also campaigned on behalf of the Birmingham 6 and attended their Appeal trial in 1990. O'Riordan served between 1970–1983 as General Secretary of the Communist Party of Ireland; and from 1983–1988 he served as National Chairman of the party publishing many articles under the auspices of the CPI. O'Riordan staunchly pro-Soviet direction of the party led to a number of members leaving to form the Eurocommunist Irish Marxist Society.