Stagg was elected in 1979 to represent the Celbridge area on Kildare County Council for the Labour Party, serving until 1993.

In January 1993, in the Fianna Fáil–Labour Party coalition government formed after the 1992 general election, he was appointed as Minister of State at the Department of the Environment, with special responsibility for Housing and Urban Renewal.

In December 1994, in the Rainbow coalition government, Stagg was appointed as Minister of State at the Department of Transport, Energy and Communications, serving until June 1997.

[2] In 1994, while Minister of State, Stagg became the subject of a major press scandal after gardaí had found him the previous November loitering in an area of Dublin's Phoenix Park used by male prostitutes.

[5][6][7] According to The Independent, public anger was largely focused on the member of the gardaí who leaked details to the media despite no crime being committed.