text: After the Troubles broke out in Northern Ireland in 1969, factions in both the IRA and Sinn Féin were divided over abstentionism and the left-wing faction's socialist politics.  The republican movement divided into Official and Provisional movements in 1969/70, but Costello remained with the Officials, owing to a greater commitment to left-wing politics.  He served as vice-president of Official Sinn Féin and as a staff officer in the Official IRA.
  He was the Official IRA's Director of Operations.
  Costello was opposed to the 1972 ceasefire and started to clash openly with the leadership, in particular with Eoin Ó Murchú. Costello was subjected to court martial in 1974. Brigid Makowski, who was called to testify at his court martial in Mornington in County Meath, remarked that "Jesus could have testified on Costello's behalf and it wouldn't have changed the verdict." He was dismissed from OSF in 1974 after the OSF leadership blocked his supporters from attending the party convention.
  He stood again in the 1974 local elections and topped the poll for the Wicklow County Council and the Bray Urban District Council.
  Founds INLA and IRSP
  At a meeting in the Lucan Spa Hotel near Dublin, on 8 December 1974, the Irish Republican Socialist Party was formed by republicans, socialists, and trade unionists with Costello as the chairperson.
  At a private meeting later the same day, the Irish National Liberation Army was formed with Costello as the Chief of Staff, although its existence was to be kept secret for a time.  The new grouping intended to combine left-wing politics with the "armed struggle" against British security forces in Northern Ireland.