In June of 1989, Proinsias was elected to the European Parliament for the Dublin constituency. He was a Vice Chairman of the Regional Affairs Committee of the Parliament and served in the European Parliament until 1992.

In that same year, he resigned from the Workers Party and helped found the Democratic Left. He was elected Leader of Democratic Left at the party's founding conference in March 1992 and was re-elected to that position twice, until Democratic Left merged with the Labour Party in 1999. It was at this period that he was elected President of the Labour.

Proinsias served as Minister for Social Welfare in the 'Rainbow' Government (Fine Gael/ Labour/Democratic Left) from 1994 to 1997, initiating a wide range of reforms, including the introduction of the country's first National Anti-Poverty Strategy. He also established the Commission of the Family and the National Pension Policy Initiative. Both have since reported with far-reaching recommendations on how to provide better support for families and for pensioners, some of which have since been implemented.

He was re-elected to the European Parliament for the Dublin Constituency in 1999. He is a Member of the Employment and Social Affairs, the Foreign Affairs and the Petitions Committees in the European Parliament, as well as the Tobin Tax intergroup, and the Full Employment Association. He is Chair of the P.E.S. Working-Group on Public Services. Proinsias is also a Member of the Spinnelli Group which consists of Socialist Group Members promoting new forms of governance for the European Union based on the idea of a Federation of Nation States (referred to as 'New Federation').