This is an overdue first biography of one of the most influential figures in Irish Labour in the first half of the 20th century. O'Brien was a delegate to the Irish Congress of Trade Unions for more than 30 years and had the distinction of being four times president of Congress. His name is mainly associated with that of Ireland's then-largest trade union, the ITGWU, which he helped to build after 1916, and of which he was General Secretary for over twenty years. That~position led to a celebrated clash with Jim Larkin. As a member of the Dáil he won respect for his speeches and interventions, and he was also respected for his stance during the Troubles; he shared imprisonment with both de Valera and Michael Collins and retained their friendship. He was mainly responsible for Connolly's return to Ireland, and later for preserving and publishing Connolly's writings. On retirement, O'Brien's interest in history led to his editing the two volume Letters of John Devoy. O'Brien left a corpus of letters, diaries, and notebooks to the National Library of Ireland. This book is based on research of that material and interviews over many years.