Una O'Dwyer became First Secretary (General Co-ordination, ANTICI Group and coordination responsibilities for the 1979 Presidency) at the Irish Permanent Representation to the European Communities in Brussels in 1977. She then became, in 1982, First Secretary of the Department of Foreign Affairs (Overseas Development Aid) and was responsible for relations with international organisations such as UNCTAD and UNIDO, which included chairing EC co-ordination meetings and negotiating on behalf of EC during the 1984 Presidency. In 1986, she became an Official in the Secretariat General of the European Commission in the Directorate for Relations with the Parliament, with responsibility for implementation of the new legislative procedures established by the Single European Act, as well as following the EP Committee for the Environment, Public Health and Consumer Protection. Between 1995 and 1999, she was Head of a new sector created to manage the new legislative procedures flowing from the Maastricht Treaty, in particular co-decision and, from 1999 to 2009, Head of a new unit "Management of the codecision procedure", created to follow the codecision procedure after Amsterdam, with additional responsibility from 2005 for the newly-created GRI secretariat (Groupe des relations inter-institutionelles) for interinstitutional relations, in particular with regard to the implementation of the Lisbon Treaty. She left the Commission in 2009.