In 1982, O'Sullivan joined the Democratic Socialist Party (DSP), a small party founded by Limerick TD Jim Kemmy, who had previously been a member of the Labour Party. There had been no political tradition in her family — her parents had supported different parties — and her choice of party was based on her support for Kemmy's anti-nationalist stance on Northern Ireland, and his advocacy of family planning services and a pro-choice approach to abortion. Family planning was deeply controversial in Ireland from the 1970s to the 1990s, particularly in Limerick, where Kemmy had lost his Dáil seat at the November 1982 general election, after being denounced by the Catholic Church for his opposition to the Eighth Amendment of the Constitution of Ireland. Those such as O'Sullivan who were involved in the family planning services which Kemmy had helped found were labelled "Kemmy's Femmies".