Career
O'Donnell began his career at Arsenal but left without making an appearance for the first team. He then signed for Falkirk and became a regular in their side. However, he rejected a contract from Falkirk in summer 2007, instead moving to.
Having rejected a new contract offer, he signed a two-year deal with Cork City on 21 February 2009. He signed for his local club, Galway United, in the weeks leading up to the 2010 League of Ireland season. He made 28 appearances for the Galway scoring eight goals.
In January 2011, O'Donnell signed for the League of Ireland champions. On 22 February 2011, O'Donnell scored in his first match for the Shamrock against in the Leinster Senior Cup. In August 2011, O'Donnell scored the winning goal from a penalty at Partizan Belgrade as Rovers became the first Irish club to reach the group stages of the UEFA Europa League He signed for on 21 December 2012.
Stephen suffered a very serious injury while playing for against former club on 18 April 2014. A scan revealed that he tore his anterior cruciate ligament and his posterior cruciate ligament and would be out for the remainder of the season. As a result, were crowned premier league champions for the first time in 19 years, winning the league by two points over Cork City who led going into the match.
It was O'Donnell's first start since returning from injury and he was awarded man of the match after an impressive display. International O'Donnell has been capped by the Republic of Ireland national under-21 football team and the under-23s.