William O'Brien (23 January 1881 – 31 October 1968) 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.