He was educated in Christian Brothers Schools in Drogheda and Castleknock. Donegan was first elected as a Fine Gael TD in 1954. He lost his seat at the following election but reclaimed it in 1961. In the Fine Gael-Labour coalition of 1973 Donegan was appointed Minister for Defence.
In October 1976 Donegan made a contoversial speech on an official visit to a military base. He referred to President Cearbhall ?D?aighs refusal to sign tough anti-crime legislation as a 'thundering disgrace'. The Taoiseach, Liam Cosgrave, refused Donegan's resignation. Instead ?D?aigh resigned as President of Ireland. The whole episode badly damaged the governments reputation.
In 1976 Donegan became Minister for Lands. He also had a short term as Minister for Fisheries in 1977. Donegan retired from politics in 1981.