Don O'Riordan, who could play as a central defender or in midfield, began his career as an apprentice with Derby County, turning professional in May 1975. His league debut came on March 23, 1977, as a 24th minute substitute for Charlie George in a 0-0 draw away to Tottenham Hotspur. He played twice on loan to Doncaster Rovers in January 1978 and struggling to make an impact at Derby, moved to NASL side Tulsa Roughnecks.
He returned to England in October 1978, joining Preston North End for a fee of £30,000. He made 158 league appearances for Preston - being voted Player of the Year in 1981/82 - before he was on the move again, joining Carlisle United in August 1983, again for a fee of £30,000.
He had two seasons as a regular at Carlisle, scoring 18 goals from 84 league appearances, earning a £55,000 move to Middlesbrough in August 1985. Despite missing only one game the following season, O'Riordan was released amidst Middlesbrough's financial crisis, joining Grimsby Town in August 1986. As well as playing 86 league games, scoring 14 goals, or Grimsby, he also acted as assistant to Grimsby manager Bobby Roberts.
In February 1993 O'Riordan joined Torquay United and that summer took over as manager. In his first season as manager, Torquay made the Division Three play-offs, followed by a rare season of mid-table safety. The 1995-96 season started badly for Torquay, and O'Riordan was sacked after an 8-1 defeat at home to Scunthorpe United in the October, with future Torquay forward Andy MacFarlane scoring 4 goals. His legacy to the club was only discovered later: he had signed Rodney Jack before his departure. He joined Scarborough as a player in December 1995, but played just once to bring down the curtain on his playing career.
He had a brief spell at Gloucester City and a loan spell at Dorchester Town before being appointed as manager of Irish side Galway United in the summer of 1997. Despite his age he did turn out on a few occasions for the Tribesmen. He was sacked by Galway as part of an economy drive in May 2001.
In July 2001, he was appointed as manager of Sligo Rovers, leaving in 2004. He has subsequently been added to the Sheffield United coaching staff under manager Neil Warnock, whom O'Riordan played for when Warnock was in charge of Notts County. O'Riordan is part of the club's Youth set-up and is currently in China where he is setting up a Youth Academy for United so that they can tap into the ever growing Asian football market.