O’Brien moved with his family to Dublin in 1897, and quickly became involved in the Irish Socialist Republican Party (ISRP). O’Brien is described as a very significant figure in the ISRP.
A close friend and associate of James Connolly, O’Brien helped establish the Irish Transport and General Workers’ Union in 1909, and was instrumental in the Dublin Lock-out strike in 1913.
A member of the Irish Neutrality League, and Anti-Conscription Committee, during World War I, O’Brien was interned on several occasions. During one of these instances, he stood in the 1920 Stockport by-election, but was refused a release to campaign in it.
With the formation of the Irish Free State, O’Brien was elected as Teachta Dála (TD) for Dublin South at the 1922 general election, and again for Tipperary in June 1927 and again in 1937.
An important figure in the Labour Party in Ireland in its formative days, O’Brien resisted James Larkin’s attempt to gain control of the Party on release from prison. Taking Larkin to court over his occupation of ITGWU headquarters, the Larkin-O’Brien feud resulted in a split within the labour and trade union movements, and the formation of the Irish Congress of Trade Unions.
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.
Active in politics and the trade union movement into his 60s, O’Brien retired in 1946 and died on 31 October 1968.
Image | The Irish Trade Union Congress, including (standing): James Connolly, far left; William O’Brien, 2nd left; and James Larkin, 2nd right (seated)