He had joined the Gaelic League by 1910 when working in London for the civil service, as had Michael Collins the previous year.
He was a member of the Irish Volunteers and of the Irish Republican Brotherhood.
He and Austin Stack had been on their way to meet Sir Roger Casement at Banna Strand in County Kerry in 1916 when they were arrested by the British authorities on Easter Saturday.
They spent Easter Week in Tralee Barracks and in solitary confinement on Spike Island, County Cork; they were then held with Terence MacSwiney, Arthur Griffith and others in Richmond Barracks before being sentenced to penal servitude for life.
At the 1921 Irish elections he was elected for the constituency of Kerry–Limerick West.
He opposed the Anglo-Irish Treaty and voted against it.
He refused an offer of the Ministry for Posts and Telegraphs if he would switch to the pro-Treaty side.
Having been sworn to non-violence – together with Richard Mulcahy – by the Augustinians, he did not join the anti-Treaty forces.