Before becoming a full-time public representative, O'Donnell worked as an accountant. He is a nephew of Tom O'Donnell, a former Minister and TD for Limerick East.
In October 2007, O'Donnell was appointed party Deputy Spokesperson on Finance, with special responsibility for Freedom of Information, Procurement Reform and the Office of Public Works. As Deputy Spokesperson, O'Donnell was given the full Finance portfolio on an acting basis by Enda Kenny on 14 June 2010, when Kenny sacked Richard Bruton.[3] O'Donnell subsequently supported Richard Bruton's leadership challenge to Enda Kenny. Following Kenny's victory in a motion of confidence, O'Donnell was not appointed to the front bench. In October 2010, he was appointed as party Deputy Spokesperson on Enterprise, Trade and Innovation, with special responsibility for Enterprise and Employment.