O'Donnell played youth football for Newcastle in County Galway before joining Arsenal as a trainee in the summer of 2002 after completing his Junior Certificate exams.

[2] While he credits the coaching he received during this time with giving him greater confidence on the ball, he has expressed regret at moving to Arsenal at such a young age, feeling that if given the choice again he would have stayed at home, completed his Leaving Certificate exams and joined a team in the League of Ireland.

He had a greater desire to be played centrally in his natural position, and after receiving no assurances about this from manager John Hughes, O'Donnell rejected a new contract in summer 2007.

[10] In August 2011, he 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.

[12] Prior to the match, he was involved in an attempt by senior players to secure a bigger bonus from the fan-owned club by threatening not to play the game.

O'Donnell signed a new two-year contract before the 2017 season but it ended in disappointment with rivals Cork City completing a League and Cup double.

[17] O'Donnell was announced as the head coach of St Patrick's Athletic on a two-year contract on 31 August 2019, his first job in senior management, following the resignation of Harry Kenny.