[1] While rarely dominating the political spotlight, O'Brien was incredibly powerful and influential behind the scenes, maintaining a firm grip over Ireland's trade unions for many decades.

[7] In 1908, believing in the importance of organising unskilled labourers, he supported the leadership of James Larkin in the formation of the Irish Transport and General Workers' Union.

Together, O'Brien, Larkin and Connolly established the Labour Party of Ireland in 1912, in their minds as the "political wing" of the Irish Trades Union Congress.

[8] The three men's leadership would also be instrumental in the Dublin Lock-out strike in 1913, quite possibly the most significant trade union action of its era.

They were held in Richmond Barracks in Inchicore with most of the other captured rebel leaders such as Thomas McDonagh, Eamonn Ceannt and Major John MacBride.

In the aftermath of the Rising, O'Brien was one of the few prominent labour leaders still remaining, with Connolly now dead and Larkin having departed for the United States in 1914 after the lockout.

A member of the Irish Neutrality League, and Anti-Conscription Committee, during the World War I, O'Brien was interned on several occasions by the Dublin Castle government.

They did, however, work together to draft the Democratic Programme, a document which had been intended to guide how the newly formed Irish state would operate.

During the ensuing Irish Civil War between pro and anti-treaty forces, the Labour movement campaigned for peace between both sides but could not halt the violence.

In 1923 Larkin returned from the United States and attempted to resume command of the entire Irish labour movement despite a decade's absence and not being present for the revolutionary period.

[12] In 1930, O'Brien sought to have Leon Trotsky granted asylum in Ireland, but the head of the Free State government, W. T. Cosgrave, refused to allow it.