Hayes, (Francis) Mahon (Joseph) (1930–2011), lawyer and diplomat, was born on 2 March 1930 at Verdun, Wellington Road, Cork, the second of three sons of Francis Mahon Hayes (d. 1968), official with the Munster and Leinster Bank, and his wife Aileen (née Walsh). The family moved with his father's work postings, but Mahon, though educated at CBS Thurles, always regarded himself as a Corkman (especially in hurling terms). After studying politics and economics at UCD, he graduated BA (1951) and studied law at the King's Inns. Called to the bar (1952), he practised on the southern circuit and contributed law reports (1953–4). On 31 July 1958 at University Church, St Stephen's Green, Dublin, he married Kathleen O'Donoghue, whom he met when visiting Castletownbere in 1950, where his father was then bank manager; they had three daughters and a son.
After working for the land registry, he was appointed assistant legal advisor at the Department of Justice (1957–65). While assistant legal advisor and first secretary at the Department of External Affairs (1965–70), he participated in negotiations leading to the Anglo–Irish free trade agreement (1965) and the double taxation treaty with Switzerland (1966). Promoted to legal advisor (1970), he also served as counsellor (1970–75) and assistant secretary (1974–7) in the department. Although not trained in international legal procedure and norms, he was at the centre of Ireland's international legal and diplomatic affairs for the next three decades.
In November–December 1970, as political violence increased in Northern Ireland, Hayes advised Taoiseach Jack Lynch (qv) on the diplomatic implications of Ireland's derogating or withdrawing from the European Convention on Human Rights (ECHR) if the government implemented detention without trial to deal with the growing threat to public order in the Republic. Somewhat ironically, soon afterwards Hayes was closely involved in Ireland's case during the 1970s against the UK before the European Commission of Human Rights and then the Court of Human Rights, challenging the implementation of internment without trial in Northern Ireland in August 1971. While the office of the attorney general led preparation of the legal complaint on the detention and treatment of suspects by British authorities in Northern Ireland, the strategy and implementation of the case was guided by Hayes, who acted as Ireland's formal agent before the court, advising taoisigh Lynch and Liam Cosgrave and working with three successive attorneys general. Hayes's 1970 assessment of the legal particulars in their international political context accurately prefigured the eventual judgment rendered by the European Court of Human Rights in 1978, and he later recalled his considerable satisfaction at this.
From 1971 he was a member of the Irish delegation that negotiated Ireland's accession to the EEC (1973), and was among twenty Irish diplomats and civil servants awarded commemorative medals by Lynch marking the achievement. Turning his attention again to Northern Ireland, he participated in the tripartite Sunningdale talks in 1973, and was highly regarded, for both his gentlemanly conduct and professional skill, by British legal and diplomatic negotiators. He was also the key figure in the Irish delegation to the Third United Nations Conference on the Law of the Sea (1973–82) (also representing Ireland in 'pre-conference' sessions), which established a comprehensive legal settlement governing the world's oceans, the seabed, resource management and extraction (aquatic and mineral), environmental preservation, and the rights of sovereign states to freedom of navigation. As deputy leader under Con Cremin (qv) of the Irish delegation for the first eight sessions (1971–9), before leading it for the final three sessions (1980–82), Hayes was spokesman for a group of thirty states on the contentious issue of delimiting sovereign boundaries at sea between contiguous states. Involving almost 5,000 delegates, working in six official languages, the conference, the most complex in international diplomatic and legal history, delineated the establishment of 'exclusive economic zones', enmeshing states in an elaborate interlocking dispute resolution and enforcement regime, novel in international law. Hayes's reputation for fairness and honesty stood Ireland in good stead as, working on a consensual basis, he led the 'margineers' group', facilitating compromise on delineating continental margins with respect to oceanic ridges and other submerged elevations. At the UN and in bilateral negotiations with the UK, he expertly managed Ireland's claim to the uninhabited Rockall islet in the north Atlantic, a contentious issue between Ireland and the UK (as well as Iceland and Denmark) from the 1970s, as seabed mineral and energy extraction became increasingly viable. Consultation on policy priorities for Ireland at the conference, and their reconciliation across and between domestic interest groups and government departments, marked the endeavour as one of the most complex ever undertaken in Ireland. Hayes later published a self-effacing account of the conference, The law of the sea: the role of the Irish delegation at the third UN conference (2011).
Committed to supporting emergent norms in international human rights law, Hayes lobbied the Department of Justice and attorney general's office to accede to (1968) and ratify (1976) the UN genocide convention of 1948, seeking thereby to improve Ireland's international reputation and diplomatic effectiveness. He was a key figure in managing overlapping political-judicial issues in Anglo–Irish, European and international forums, deeply committed to the 'importance of internationally agreed rules of conduct among states' (O'Brien, Bar Review). His calm, compassionate manner served his diplomatic career well, as did his keen legal mind and strategic intelligence in contexts where cooperation necessitated persuasion and compromise.
Hayes was appointed Ireland's ambassador to Denmark, Norway and Iceland (1977–81), perhaps owing to his knowledge and diplomatic experience of territorial issues regarding Ireland's maritime sovereignty, particularly the delineation of boundaries regarding Rockall, then playing out at the third UN conference. (They were subsequently fully resolved within the legal regime that emerged from the conference, which the four disputing states all ratified.) He then served as Ireland's permanent representative to the UN and other international organisations in Geneva (1981–7), notably the WHO and the International Labour Organisation. He was vice-president of the UN Commission on Human Rights (1982–4), and during his career at the UN strongly denounced abuses in Cambodia, South Africa, Palestine and Central America. In 1986 he was elected by secret ballot of the UN general assembly to the highly prestigious 35-person UN International Law Commission (1987–91), the first Irish-nominated candidate so elected, and urged consensual harmonisation of international law regarding war crimes and crimes against humanity. Returning to Dublin as deputy secretary of the department (1987–9), he led its economic division, responsible for Ireland's European affairs during the implementation of the Single European Act (1987) in preparation for the European single market (1992). He was next appointed Ireland's permanent representative to the UN in New York (1989–95), and lobbied there in support of President Mary Robinson's campaign to highlight international human rights abuses, specifically her activism at the UN on the 1992 humanitarian crises in Somalia.
Appointed Irish representative to the Permanent Court of Arbitration's panel of arbitrators, Hayes also participated in expanding the regime of the nuclear non-proliferation treaty (1968) as the Soviet Union collapsed (1990–92), in securing EU–UN cooperation on biological and chemical arms-control regimes in the context of linking them to emergent human rights norms, in the GATT negotiations (forerunner of the WTO), and in the work of the Council of Europe and various attendant bodies and committees.
On reaching the age of 65 in 1995, he officially retired from Foreign Affairs but remained active in public life. In April 1995 he was appointed one of three conveners to the constitution review group, chaired by T. K. Whitaker (1917–2017); the group's report (July 1996) is regarded as the most comprehensive and scholarly analysis of the Irish constitution in its political and legal contexts. During Ireland's presidency of the EU council of ministers (July–December 1996), Hayes advised the Irish government, assisting with preparations for the intergovernmental conference that produced the 1997 Amsterdam treaty. He was a member of the Department of Foreign Affairs sub-committee formed (May 1997) by Minister Dick Spring to promote Mary Robinson's candidacy for UN high commissioner for human rights, and was one of Ireland's representatives on the European Convention (1999–2000) that drafted the EU charter of fundamental rights. In 2001 he was appointed to the task force on emigration policy by Minister for Foreign Affairs Brian Cowen, and acted as a consultant to the Palestinian Authority in the drafting of a constitution for a future Palestinian state.
After a short illness, he died on 27 June 2011 at the family home in Goatstown, Dublin, and was buried in Kilmashogue cemetery. He was widely praised, in Ireland and abroad, for his abilities and modesty. The NUIG School of Law established The Mahon Hayes memorial prize in the law of the sea in 2012.