In 1944 he was founding secretary of the Liam Mellows Branch of the Labour Party. This branch became infamous for what was regarded during the period as its controversial nature and became an intractable enemy of Branch Chair Timothy Quill. The branch was initially established by former members of the Curragh Camp’s Communist Group, including Bill Nagle and Jim Savage. During this time, the IT&GWU had disaffiliated from the Labour Party and the National Labour Party was established on the basis that communists had infiltrated the Party. Quill, who was made branch chair by the Labour Party, allegedly had O’Riordan and his fellow members expelled, with the branch being dissolved. O’Riordan later accused Quill of anti-Semitism and both Quill and T.J Murphy of “red-baiting”. In 2001, O'Riordan would claim that any attempt to raise the issue of defence of communist Spain ""was shouted down at Labour Party Conferences"". In 1945 he was a founding secretary of the Cork Socialist Party, whose other notable members included Derry Kelleher, Kevin Neville and Máire Keohane-Sheehan (Máire Keohane-Sheehan was the sister of O'Riordan's wife Kay Keohane-O'Riordan) Thomas Murray.