Costello, Séamus (1939–77), republican socialist, was born at Old Connaught Avenue, Bray, Co. Wicklow, son of John Costello, farmer and cattle-dealer, and his wife Elizabeth. He attended the Christian Brothers' college at Monkstown, before taking a job as a car salesman in central Dublin. Costello earned the title ‘boy general’ by burning down the courthouse in Magherafelt, Co. Londonderry, as part of the IRA's border campaign in the late 1950s. After the collapse of this campaign, he became a full-time political activist with Sinn Féin, advocating an end to the party's traditional policy of abstentionism. In 1967 Costello won a seat on Wicklow county council (and Bray urban district council), and the following year secured over 2,000 first-preference votes in a dáil by-election (March 1968). He quickly established a reputation as an energetic and committed local representative, actively involved in setting up housing-action committees, tenants-rights associations, and community pressure groups. The outbreak of violence in the North in 1968–9 temporarily diverted his attention from parliamentary politics, but Costello subsequently contested the 1973 and 1977 general elections. Within the republican movement he associated with the radical left, arguing for a need to combine socialism, community activism, parliamentary politics, and military struggle.
After the republican split in 1970, he became vice-president of Official Sinn Féin, and at the same time director of operations of the Official IRA. He vehemently opposed the Official IRA cease-fire declared on 29 May 1972, leading eventually to his dismissal from the Official republican movement (1974). Ironically, Costello enjoyed considerable political success at this time, with his election as chairman of Bray urban district council in 1973–4, and a poll-topping performance in the 1974 Wicklow local elections. After failing to be reinstated into Official Sinn Féin, Costello and his republican allies formed a new political party, the Irish Republican Socialist Party (IRSP), on 8 December 1974. The party received a major boost when shortly afterwards the former MP Bernadette McAliskey became a member. A military wing, the Irish National Liberation Army (INLA), also emerged, with Costello acting as chief-of-staff. This new movement sought to reestablish the link between the national question and revolutionary socialism. Costello believed that ending the partition of the island was an essential precursor to creating a genuinely socialist republic, and he hoped to develop a ‘broad front’ of anti-imperialist groups committed to this end. He also believed, unlike his former colleagues in Official Sinn Féin, that British interests in Ireland needed to be challenged militarily. This combination of political and military activities created tension within the IRSP, and led to confrontation with other republican groups. Early in 1975 a bloody feud erupted between the INLA and the Official IRA, resulting in a number of deaths, and Costello himself narrowly escaped injury in an assassination attempt outside Waterford city in May 1975, before an uneasy truce was arranged. Later that same year, Bernadette McAliskey, concerned at the increased militancy of the new movement, resigned from the IRSP, taking over half of the Árd Chomhairle members with her.
Early in 1976 Costello's ambitions suffered a further set-back after the arrest of IRSP members in connection with the Sallins train robbery. Amid allegations of Garda brutality (substantiated by Amnesty International), Nicky Kelly and others were eventually convicted by the special criminal court in 1978. The Provisional IRA later claimed responsibility for the robbery, and Kelly finally received an official pardon; but this episode, combined with the Official IRA feud and internal divisions, seriously damaged both the IRSP and INLA. In July 1976 Costello was replaced as INLA chief-of-staff by South Derry man Eddy McNicholl, although he still wielded considerable influence within the movement, retaining his position as chairman of the IRSP. On 5 October 1977 a lone gunman shot Costello dead on Northbrook Avenue, off the North Strand in Dublin. Although the police never arrested anyone for the killing, on 4 June 1982 Jim Flynn, a leading member of the Official IRA and the man widely suspected of carrying out the assassination, was shot dead in Dublin by the INLA. Costello was the first party leader to be killed in the history of the state, and thousands of people, including a number of TDs, attended his funeral in Bray. His death deprived the IRSP of its only charismatic leader, while the INLA gradually disintegrated as a result of factional disputes. Costello was survived by his wife, Maelíosa, and four children.