text: Within days of its founding, the fledgling Irish Republican Socialist Movement was embroiled in a bitter feud with the Official IRA. The feud resulted in the attempted assassination of Official IRA leader Sean Garland who was wounded in an INLA attack near his home in the Ballymun area of Dublin (Garland was wounded six times but survived the attack). Before a truce was reached, three members of the IRSP were dead.  Later that same year, Bernadette Devlin McAliskey resigned from the IRSP over the failure of a motion that would have brought the INLA under IRSP control, taking over half of the Ard Chomhairle members with her.
  In July 1976 Costello was replaced as INLA chief-of-staff by South Londonderry man Eddy McNicholl, although he still wielded considerable influence within the movement, retaining his position as chairman of the IRSP.
  Death
  Despite the truce, Costello was shot dead with a shotgun as he sat in his car on Northbrook Avenue, off the North Strand Road in Dublin on 5 October 1977 allegedly by a member of the Official IRA, Jim Flynn, who happened to be in the area at the time. The Official and Provisional IRAs both denied responsibility and Sinn Féin/The Workers' Party issued a statement condemning the killing. Members of an opposing INLA faction in Belfast also denied the killing. However, the INLA eventually deemed Flynn the person responsible, and he was shot dead in June 1982 in the North Strand, Dublin, very close to the spot where Costello died.