Ryan played his youth football for Belvedere FC. Ryan began his senior career at Huddersfield Town in July 1994 but did not make his first team debut until September 1996 when he came on as a sub in a 3-1 win over Ipswich Town.
After making just 15 Appearances for The Terriers, Ryan joined Millwall in 2000, costing the club £10,000. He made his debut in a 0-0 draw with Southend United. In the 2000-01 season he only missed four games for Millwall as they won the Football League Second Division title. The following season, Ryan played 32 league games as Millwall made the First Division play offs, losing to Birmingham City.
At the end of his contract, Millwall offered him another one-year deal, which he declined in favour of a three-year deal with Bristol Rovers, despite dropping down two divisions. He had played 226 league games for Millwall. His last appearance in a Millwall shirt happened to be the 2004 FA Cup Final against Manchester United where he was marking Cristiano Ronaldo.
Computer game Championship Manager played its part in Ryan's transfer. Utilising real-life statistical data, the game revealed to Ryan Molesworth, the son of Bristol Rovers scout Paul Molesworth, that Robbie Ryan's contract was expiring. Rovers then became interested in signing the player, which after negotiations, actually took place.
In the 2004-05 season, Ryan made 40 league appearances for Bristol Rovers. In 2005-06 he played only 17 games, none at all since November. On February 1, 2007 Bristol Rovers terminated Ryan's contract.
Ryan has represented Ireland at schoolboy and youth levels playing in the FIFA World Youth Champions in Malaysia for the Republic of Ireland who came 3rd and has also picked up 12 U21 caps.