[1] A longtime senior member of the Irish Republican Brotherhood (IRB), he served as its president from May 1917 until May 1918 when he was one of a number of nationalist leaders arrested for his alleged involvement in the so-called German Plot.

McGarry participated in the 1916 Easter Rising as an aide-de-camp to Tom Clarke[2] and sentenced to eight years penal servitude for his role in the failed rebellion.

[7] He was only imprisoned a short time when he took part in the famous escape from Lincoln Jail with Seán Milroy and Éamon de Valera on 3 February 1919.

[11] He, like the majority of those in the Irish Republican Brotherhood, supported the Anglo-Irish Treaty and was involved in debates against de Valera during the controversy, most especially discussing the status of Sinn Féin as a political entity.

As one anti-Treaty volunteer told Ernie O'Malley, "Seán McGarry was often drunk in Amiens St. and the boys wanted to shoot him and the Staters there but I wouldn't let them..."[13] On 10 December 1922, shortly before the first meeting of the Free State parliament, a fire was deliberately set by irregulars (anti-Treatyites) at his family home.