Born in Dolphin's Barn, Merrigan grew up in poverty after the death of his father. He left school at 13 to work, and when he was 15 he started to work in the Rowntree-Mackintosh chocolate factory, where he worked for the next 20 years. He became an Amalgamated Transport and General Workers Union (ATGWU) shop steward in the 1930s, rising in 1960 to become the Union's National Secretary in the Republic of Ireland, a post he held until 1986. He was president of the Irish Congress of Trade Unions in 1985. As a trade unionist, he opposed national wage agreements as doing nothing to improve the workers' share of national income.
Merrigan joined the Irish Labour Party in the 1950s, and unsuccessfully contested the 1954 general election in Dublin South-West. He did not stand again until the Dublin South-West by-election in 1970, when he narrowly missed holding the Labour-held seat, due to the intervention as an independent candidate of Cora Dunne, widow of Sean Dunne, whose death had triggered the by-election. He opposed the party's formation in 1973 of a coalition government with Fine Gael, and as a left-wing critic he was expelled from the party in 1977. He contested the 1977 general election as an independent candidate in Dublin Finglas, but was not elected.
Dr. Noel Browne, another left-wing critic who had also been expelled from Labour, did win a Dáil seat at that election, and they were co-founders of the new Socialist Labour Party, of which Merrigan became chairman. The new party was short-lived, and although Merrigan stood at two further Dáil elections, his share of the vote was tiny.
He remained a vocal critic of the Labour Party's participation in coalitions, and supported protests against the PAYE tax system. He also served as a member of the Employment Appeals Tribunal.
Merrigan died in hospital in Dublin on 15 June 2000, aged 78, after collapsing earlier that day at the ATGWU's conference in Malahide, County Dublin. His wife Rose had died some years before, but he was survived by their daughter and two sons — the elder of whom, also called Matt, is an official of the Services, Industrial, Professional and Technical Union (SIPTU).
- "A socialist visionary and trade union pragmatist". The Irish Times. 17 June 2000. Retrieved 2008-03-09.
- "Tom Kitt Pays Tribute to Matt Merrigan". Department of Enterprise, Trade and Employment. 2000. Retrieved 2008-03-09.
- "Matthew Merrigan's electoral history". ElectionsIreland.org. Retrieved 2008-03-09.
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