Determined to gain Irish independence from Britain, he took part in the Easter Rising (1916). Elected to the British Parliament in 1918 as a Sinn Fein member, he became Minister for Local Government in the self-declared government of the Dáil Éireann in 1919. He reluctantly accepted the Anglo-Irish Treaty creating the Irish Free State. He was President of the Executive Council of the Free State from 1922–1932, during which time the international standing of the new state was greatly enhanced. He was leader of the opposition Fine Gael party in the Dáil Eireann (1933–44). He was the father of Liam Cosgrave (1920–2017), who in turn became leader of Fine Gael (1965–77) and later Taoiseach (Prime Minister) of the Republic of Ireland (1973–77).