O’Brien moves with his family to Dublin in 1897, and quickly becomes involved in the Irish Socialist Republican Party (ISRP). He is described as “a very significant figure in the ISRP” by ISRP historian David Lynch. He is a member of the Socialist Party of Ireland, serving on its executive.
A member of the Irish Neutrality League and Anti-Conscription Committee during World War I, O’Brien is interned on several occasions by the Dublin Castle government. During one of these instances, he stands in the 1920 Stockport by-election, but is refused a release to campaign in it.
An important figure in the Labour Party in Ireland in its formative days, O’Brien resists 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 results in a split within the labour and trade union movements, and the formation of the Irish Congress of Trade Unions.
Active in politics and the trade union movement into his 60s, O’Brien retires in 1946 and dies on October 31, 1968.