Born in Mallow, County Cork, McDonnell began working in the local Érin Foods factory.
He joined the Irish Transport and General Workers' Union (ITGWU), and in 1974 was elected as secretary of his local branch.
He focused on supporting union members in the food processing, local authority and health industries, and gradually came to national prominence.
Because SIPTU, at the time, expected leaders to retire when they reached the age of sixty, he committed from the start to serving a single five-year term.
After retiring from his union posts in 2003, the ICTU nominated him to serve on the board of Fáilte Ireland, the tourism development agency.