Mahon Hayes, who died on June 26, had a distingui-shed career in the Department of Foreign Affairs as a lawyer and ambassador. In retirement he wrote an account of the contribution of the Irish delegation, of which he was the lynchpin, to the negotiation of the United Nations Law of the Sea Convention concluded in 1982. It was launched in April by Tanaiste and Minister for Foreign Affairs Eamon Gilmore.
Francis Mahon Hayes, to give him his full name, was born on March 2, 1930, the second son of a Cork-born bank manager stationed in Thurles. He was educated at CBS Thurles, UCD and King's Inns.
Called to the Bar in 1952, he practised on the Leinster Circuit. It was a lean time for barristers and he did not enhance his prospects when he reported in the Irish Law Times a foolish decision of the local circuit judge without that judge's permission.
Hayes then found employment in the Land Registry, from which he moved on to the Department of Justice and the Department of External Affairs, where as assistant legal adviser he won his spurs providing legal advice on the Free Trade Agreement with Britain concluded in 1965.
Succeeding as legal adviser in 1970, he bore with Declan Quigley in the Attorney General's Office the burden of providing legal advice on Ireland's entry to the European Community. Hayes was involved, albeit only in the largely ministerial role of agent, in the case before the European Commission of Human Rights taken against Britain arising out of the ill-treatment of detainees in Northern Ireland. It fell to him to give advice, often at short notice, on incidents near the Border and elsewhere arising out of the conflict in Northern Ireland.
While he was at a disadvantage in never having had formal training in international law, his reservoir of common sense generally led him to sound conclusions. His opinions, written in a tidy legible hand, were lucidly expressed.
Hayes had responsibility within the department for the delimitation of the continental shelf with Britain, an issue that escalated when the British government claimed that they could use Rockall and other off-shore rocks as measuring points for a median line of division. Ireland was at a disadvantage in the dispute having failed to protest at the time against the British annexation of Rockall and their appropriation in 1970 of areas of the continental shelf to which Ireland had a good legal claim.
With the negotiation deadlocked, the issue was carried into the Law of the Sea Conference, which opened in Caracas in 1973. It was a mark of the stature Hayes ultimately attained at the conference that he was selected as spokesman for the group of some 30 countries who successfully opposed the insertion of any reference to the median line in the convention. Whether this works to Ireland's advantage is still unclear.
In 1977 he transferred into the diplomatic service as ambassador to Denmark, while still attending the sessions of the Law of the Sea conference in Geneva and New York. Hayes moved from Denmark to become the head of mission at the United Nations, first in Geneva and then in New York. He was on the International Law Commission until Ireland was voted off it. Eamon Delaney was on his staff and in his memoir, Accidental Diplomat, he depicted Hayes as an uncongenial boss interested only in ministers and not in his subordinates. He was not, it is fair to add, unique in this.
Hayes ended his career as deputy secretary of the Department of Foreign Affairs. In retirement after 1995 he served on the Constitution of Ireland Review Group under TK Whitaker and contributed valuably, if rather conservatively, on the provisions dealing with international law. He also acted as consultant to the Palestinian Authority on the drafting of a constitution and advised the Institute of European Affairs on a constitution for Europe.
A tall handsome man with an understated manner, Hayes played hockey in his youth and was a keen golfer in later life.
He is survived by his wife Kathleen and four children.