Herzog, Isaac (Yitzak) Halevi (1888–1959), first chief rabbi of the Irish Free State, was born at Lomza, Poland, the only son of Rabbi Joel Herzog, rabbi of the Polish Jewish community in Paris, and Miriam Liba Herzog (née Cirowitz). With an initial education provided by his father, by the age of nine Herzog had acquired a reputation as an iliu, or distinguished thinker. His academic career, which led to his becoming a scholar in areas as far apart as law, classics, mathematics, and marine biology rather than strictly in the traditional field of Talmudic or Jewish studies, began formally in 1897 at the Sorbonne in Paris, where he studied oriental languages. He subsequently studied at Leeds in England and matriculated in the first division at the University of London (1905). In 1909 he graduated BA in classical and modern languages and mathematics, and was ordained a rabbi (1910), receiving an MA in Semitics at London University (1911), and a doctorate in literature from the same university in 1914. The following year he was appointed rabbi of the Jewish community in Belfast, where he remained for four years and began publishing works on legal and Semitic subjects and contributed to American, British, and Continental journals as well as to the proceedings of the Belfast Natural History and Philosophical Society.
In 1919 he was appointed chief rabbi of the Jewish community in Dublin, becoming chief rabbi of the Irish Free State in 1925. Scholarly, absent-minded, enormously popular, and highly regarded internationally, Herzog identified himself closely with Irish nationalism and enjoyed a close relationship with Éamon de Valera (qv), who shared his love of mathematics, and frequently visited him while in opposition in the 1920s. He also remained on good terms with W. T. Cosgrave (qv) and other members of the Cumann na nGaedheal government, and frequently made successful representations to the government in the 1920s on behalf of Jews wishing to take up employment in Ireland. The Jewish community in Ireland, over which he presided until 1936, prospered during this era (no doubt helped by Herzog's facility for diplomacy in dealing with both leaders of church and state), and he also enjoyed a close friendship with the catholic bishop of Down and Connor, Joseph MacRory (qv), who later became archbishop of Armagh.
The prosperity of the Jewish community at this time was reflected in the opening of a new synagogue in Dublin, Greenville Hall, and the extension of the Adelaide Road synagogue. Its good relations with the state were shown by the granting of public financial support for Jewish schools, and state recognition of shehitah, the traditional Jewish method of slaughtering livestock. The Jewish community was strongly represented in the business, academic, and arts worlds, with Herzog a pivotal figure in giving it greater confidence and cohesion in the new state. However, it was also necessary for him, particularly in the 1930s, to respond to incidents of overt anti-Semitism. In 1934 The Cross, published by the Passionist Fathers in Dublin, accused Jewish international finance of controlling and spreading communism, to which Herzog responded by criticising ‘all the hocus pocus engendered by fanatical imaginations reminiscent of the worst passions of the middle ages’ (cited in Keogh, 98). Likewise he complained to Archbishop Edward Byrne (qv) of the actions of Paddy Belton (qv) of the Irish Christian Front, in blaming Jews for the outbreak of the Spanish civil war, and registered his objection to the pro-Nazi sympathies of Charles Bewley (qv), the Irish envoy in Berlin. A committed social reformer, Herzog was more vocal than his catholic counterparts in decrying the poverty and squalor of Dublin's inner-city slums.
On 1 December 1936 he was selected as chief rabbi of Palestine; in the same year he was consulted by de Valera regarding the framing of a new constitution, and his own book, Main institutions of Jewish law, was published. A multi-party farewell reception was held for Herzog in Dublin's Mansion House in April 1937, where he admitted his sadness at leaving Dublin, but his inability ‘to resist the call of Zion at a turning point in the history of Israel’ (cited in Keogh, 98). Throughout the second world war Herzog was in regular contact with de Valera through a series of urgent telegrams, informing him of the Holocaust and desperately seeking help in accommodating refugees in Ireland. When visiting Dublin in September 1946, Herzog expressed concern at the country's restrictive refugee policy. He died 25 July 1959 in Jerusalem.
He married (1917) Sarah, daughter of Rabbi S. I. Hillman of London, ambassador to the chief rabbi of the British empire. They had two sons, Chaim (qv), born in Belfast, who became president of Israel 1983–93, and Jacob, born in Dublin (1921), who studied law in Canada and worked at the Israeli embassy in Washington.