William O'Brien was a politician and trade unionist in Ireland. 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. Besides his leadership in the trade unions, O'Brien was a founder, alongside James Larkin and James Connolly, of the Labour Party of Ireland. In later years a rift formed between Larkin and O'Brien that would last the rest of their lives and often divide the labour movement in Ireland.

James Larkin, sometimes known as Jim Larkin or Big Jim, was an Irish republican, socialist and trade union leader. He was one of the founders of the Irish Labour Party along with James Connolly and William O'Brien, and later the founder of the Irish Worker League, as well as the Irish Transport and General Workers' Union (ITGWU) and the Workers' Union of Ireland. Along with Connolly and Jack White, he was also a founder of the Irish Citizen Army. Larkin was a leading figure in the Syndicalist movement.