Kiernan, Thomas Joseph (‘Tommy’; ‘T. J.’) (1897–1967), diplomat, was born 9 November 1897 at 3 Moyne Road, Rathmines, Dublin, son of Francis Kiernan, clerk, and Kathleen Kiernan (neé Hurley). He had three brothers – Brian, Michael, and Brendan (who later became legal adviser to the Department of Local Government) – and one sister, Vera. He was educated at St Mary's College, Rathmines, UCD (BA, MA, both first-class honours), and London University (Ph.D.). Entering the civil service as a clerk (1916), he was promoted to surveyor of taxes and then assistant inspector of taxes, before becoming inspector of taxes in Co. Galway (1922–4). He married (1924) the singer Delia Murphy (qv); they had one son, Colm, and three daughters – Blon, Nuala, and Orla.
Also in 1924, Kiernan moved to the Department of External Affairs and was appointed secretary to the Irish high commissioner's office in London, succeeding Patrick McGilligan (qv). He held that post until 1 May 1935, when he was seconded from External Affairs and appointed director of Radio Éireann. At Radio Éireann he felt that programmes should reflect the culture of Ireland but not be too deeply ingrained in the past, commenting that ‘merely to be traditional is not to be national, certainly not constructively national . . . radio programmes should not cater for “musical drunkards”’ (Irish Press, 29 Dec. 1967). Following the removal of Charles Bewley (qv) as minister to Berlin, it was anticipated that Kiernan would fill the vacancy, but the outbreak of the second world war intervened and Kiernan remained with Radio Éireann.
In 1941 Kiernan returned to the diplomatic service and was appointed minister to the Holy See (1941–6). Kiernan and Delia Murphy kept something of a lively and bohemian legation in wartime Rome, helping refugees and displaced persons and offering memorable hospitality to all comers at the Irish legation. Kiernan developed good relations with the Irish religious communities in Rome and became friendly with Pope Pius XII, of whom he later wrote a biography. He was active in relief work in the city, and his reports to Dublin provide a vivid picture of conditions in Rome and Italy during wartime. Writing on 23 September 1944 he described how ‘the administration in Italy is chaotic, and without enough food and no transport and electric light only once in four days, the people are more down than ever. The Communist Party – apparently flush with funds – is making headway in all the towns and villages, feeding people provided they belong to the party’ (NAI, DFA P12/2, Kiernan to Walshe).
After the war, as part of the expansion of the Irish diplomatic service, Kiernan was appointed Ireland's first minister plenipotentiary to Australia (1946–50), and was promoted to ambassador to Australia in 1950. It had been rumoured that he would succeed John Dulanty (qv) as Irish ambassador to Britain, but F. H. Boland (qv) eventually succeeded Dulanty. There now arose the concurrent question of the reciprocal appointment of an Australian ambassador to Ireland. The Australian government would not send credentials to the ‘president of Ireland’, as Dublin required following the declaration of the Republic of Ireland in 1949. It would only send credentials to the ‘president of the Republic of Ireland’. Kiernan dealt with this difficult situation, but it led to an impasse over the appointment of an Australian ambassador to Ireland and, following Kiernan's return to Ireland in 1955, the appointment of his successor. The impasse lasted until 1964, when Eoin MacWhite (qv) was appointed Irish ambassador to Australia and Hugh Roberton became the first Australian ambassador to Ireland. Between 1955 and 1964 both countries were represented in their respective capitals by a chargé d'affaires.
In 1955 Kiernan returned to Europe as minister to West Germany, with personal rank of ambassador (1955–6). He served as ambassador to Canada from 1956 to 1960; his final posting was as ambassador to Washington (1960–64) during the Kennedy and Johnson administrations. He had been due to retire in 1962, but remained as ambassador until 1964. Kiernan undertook much of the preparatory work behind President Kennedy's visit to Ireland in 1963. When he met Kennedy in June 1963 to discuss arrangements, Kiernan found the president in ‘an unusual state of irritation and nervousness’, due to reports of the first Soviet female astronaut and of growing racial tension in the US. The two men discussed the presidential visit to Ireland, and when Kiernan brought up partition Kennedy, worried that he might be expected to make a public pronouncement on the subject, ‘looked as if another headache had struck him’ (Kiernan to McCann, 17 June 1963; NAI, DT S17401C). Kiernan assured him that no such statement was required and that Dublin simply ‘hoped for his continued goodwill towards a solution of the reunification of the country’.
Kiernan was a driving force in the establishment of the Irish-American Foundation, and following its establishment in 1963 by Éamon de Valera (qv) during Kennedy's visit to Ireland, Kiernan became its director. A fluent Irish-speaker, Kiernan was a member of the Folklore of Ireland Society and of the Irish Texts Society. In 1937 he was appointed external examiner in economics, economic history, and commerce through Irish by the NUI. He was also a fellow of the Royal Statistical Society.
In an obituary Kiernan was described as ‘an exact and fluent writer [who] studied the social and political situations on each of his tours of duty’ (Irish Times, 29 December 1967). He was a prolific author: his non-fiction publications include British war finance and the consequences (), A study in national finance (), A study in ecclesiastical statistics (1923), A history of the financial administration of Ireland to 1817 (1930), The Irish exiles in Australia (), and Pope Pius XII (). While ambassador to Washington he wrote a novel, The white hound of the mountain (1962). T. J. Kiernan retired in April 1964 and died 28 December 1967 at his home in Dublin; his body was donated to the RCSI.