During his period of influence, Gilmore attempted to shift the IRA to the political left, but alongside Peadar O'Donnell and Frank Ryan he was expelled for his efforts.

Despite his father primarily working for Unionist landlords, and being educated at home, George and his brothers Harry and Charlie all turned towards Irish Republicanism.

During the civil war Gilmore was captured and imprisoned, but he managed to escape custody in August 1923, the aftermath of which caused riots as the remaining prisoners were placed in solitary confinement.

[3] Following the end of the civil war, Gilmore served as the secretary of future Taoiseach Seán Lemass, [6] as well alongside Frank Aiken.

During the early 1920s Lemass, Aiken and Gilmore regularly meet with the IRA army council to represent the emerging political leadership of Irish republicanism that would coalesce as Fianna Fáil in 1926.

In October tried to escape with the help of his brother Charlie and almost succeeded, using a plot involving mock pistols wrapped in silver to intimidate the guards.

[10] Neither recognised the authority of the court, with George stating "I do not want anybody to think I excuse myself for such a charge as having arms, I am admittedly hostile to British imperialism and international capitalism".

In the aftermath Frank Aiken, former Chief of Staff of the IRA and new minister for defence went to see Gilmore on March 9 and on the next day all republican prisoners were released as part of a general amnesty.

With his close personal ties to their leadership, Gilmore had a positive view of Fianna Fáil, and at that point in time believed their goals differed little from his own and those of the IRA.

[12][3] On 14 August 1932, he and fellow Irish Republican T.J. Ryan were beaten badly, shot and wounded by plain-clothes members the Garda Síochána (Criminal Investigation Department) in Kilrush, County Clare.

Gilmore had made a last-ditch effort to save the Congress by travelling to America to seek funds from Irish-American groups but was not successful.

[17] In 1966, for the 50th anniversary of the Easter Rising, Gilmore released a pamphlet entitled "Labour and the republican movement" in which he espoused the principles of James Connolly.

[3] In the mid-1930s, Gilmore met Cora Hughes, a graduate of Celtic Studies from University College Dublin who had begun to move in left-wing circles and who had become a housing activist in the city.