On Saturday night 22 April 1916, a tense meeting in Dublin went on into the small hours to decide whether or not the Easter Rising would go ahead. Present at that meeting were Pádraig Pearse, Tomás MacDonagh, Joseph Plunkett and Seán MacDiarmada. The fifth man present at the all-night session, Diarmuid Lynch, was the only one still alive a month later. It is difficult to understand how Lynch, a member of the Supreme Council of the IRB, has been forgotten so completely.

Lynch was at the heart of plans for the Rising and was aide-de-camp to James Connolly in the GPO. Initially sentenced to death, his sentence was commuted to ten years penal servitude because he was an American citizen. However, he was released on 16 June 1917. Immediately following his release, Lynch became active again, and along with Michael Collins and Thomas Ashe, participated in the reorganisation of the IRB. After the 1917 Sinn Féin Ard Fheis, Lynch, like Collins, held three senior posts: in the IRB, Sinn Féin and in the Irish Volunteers. He was again arrested and deported to America in 1918.

Lynch was elected, although still in the US, as a TD for the constituency of Cork South-East in the 1918 elections. In America he was working frenetically as the national secretary of the FOIF (Friends of Irish Freedom) organisation, but later sharp differences arose between De Valera and the FOIF about how funds raised in America should be spent. Lynch did not take part in the Civil War, but made several unsuccessful attempts to stop it.