William X. O'Brien (ITGWU) and William O'Brien (Dominion Home Rule/IPP) were contemporaries in Irish politics early in the 20th century, but should not be confused. For other people of the same name see William O'Brien .
William X. O'Brien (23 January 1881 – 31 October 1968) was a politician and trade unionist in Ireland.
Born in Clonakilty, County Cork, O'Brien moved with his family to Dublin in 1897, and quickly became involved in the Irish Socialist Republican Party (ISRP).
A member of the Irish Neutrality League, and Anti-Conscription Committee, during the Great War, O'Brien was interned on several occasions by the Dublin Castle government. During one of these instances, he stood in the Stockport by-election, 1920, but was refused a release to campaign in it.