His father had fought in the International Brigades during the Spanish Civil War, and this greatly influenced the trajectory of O'Riordan's life.
He got a scholarship to Synge Street CBS and went on to get a BA degree in economics and politics from University College Dublin, followed by an MA in economics and labour history from the University of New Hampshire, Durham, U.S. On his return to Ireland he worked in the research department of the Irish Transport and General Workers' Union (ITGWU) and, after amalgamations, of SIPTU, the Services, Industrial, Professional and Technical Union, from 1971 to 2010.
[3] O'Riordan was a dedicated memorialist of the International Brigades in which his father had served during the Spanish Civil War.
He defended the reputation of Frank Ryan of the Brigades, who was accused by some of reneging on his anti-fascist beliefs after his capture in Spain and removal to Germany.
On the day after his death, at an away game against Dundalk at Oriel Park, a minute's silence was held in his memory and banners raised saying RIP Manus and No Pasaran, the watchword of the International Brigades.