In the aftermath of Britain’s 2016 decision to leave the EU, the Irish-American ethnic lobby remobilized to defend the Belfast/Good Friday Agreement from the threats posed by Brexit. Drawing on interviews in Washington, Belfast, Dublin and London, Cowell-Meyers and Gallaher explore how the lobby shaped US policy towards Brexit in order to prevent the reimposition of a hard border on the island of Ireland and preserve peace. The authors also consider the lobby’s approach to other contentious issues, including a united Ireland and Troubles-related investigations.