Jackson, Kenneth Hurlstone (1909–91), Celtic scholar, was born 1 November 1909 at Beddington, Surrey, younger child of Alan Jackson, stockbroker, and his wife, Lucy (née Hurlstone). He was educated at Hillcrest School, Wallington, and Whitgift grammar school, Croydon, where he developed considerable linguistic skills and won a scholarship to St John's College, Cambridge, in 1928. At Cambridge, Jackson graduated first-class with distinction in both parts of the classics tripos (1930 and 1931) and won medals for Greek and Latin verse; in 1932 he again took a first, this time in Celtic, Anglo-Saxon and Old Norse in the archaeology and anthropology tripos. This allowed him to display his mastery of the Celtic languages. He was awarded the Richard Allen studentship, which enabled him to study early Welsh poetry at Bangor under Ifor Williams and Old Irish at UCD under Osborn Bergin (qv) and Gerard Murphy (qv).
In 1934 Jackson was elected to a fellowship at St John's College, Cambridge, and appointed to a faculty lecturership in Celtic. Scéalta ón mBlascaod (1939) is a collection of folk tales from the Blaskets, many of which he had collected from Peig Sayers (qv) and had already published in the journal Béaloideas (1934, 1938). Jackson described Sayers – somewhat questionably – as ‘a woman from the middle ages’ (O'Toole, ‘The clod and the Continent’), and their encounter impressed him with the significance of oral tradition.
During the second world war Jackson served as a British censor in Bermuda, qualifying for the task in twenty-three languages. He was appointed lecturer at Harvard University in 1939, became an associate professor in 1940, and held a chair in Harvard 1949–50, before accepting the chair of Celtic languages, literature, history, and antiquities at Edinburgh University; he remained at Edinburgh until 1979. Much of his spare time during his professorial career was employed in recording the dialects of Scottish Gaelic for the Linguistic Survey of Scotland and as a commissioner for the historic monuments of Scotland.
Jackson's Early Welsh gnomic poems appeared in 1935. His Studies in early Celtic nature poetry of the same year provides translations of early medieval Irish and Welsh poems; it attempts to classify the different types of poetry habitually lumped together under this heading, and to explore their purposes and the sort of people who produced them. Cath Maighe Léna (1938) was the first critical edition of a text of Middle Irish origin; it describes the supposed ancestor of the Eoghanacht dynasty of Munster. Throughout his career Jackson hoped to produce a revised version of this edition, on which he started work in 1934 when he had just begun to study Irish; ill health finally made this impossible, but a reprint with some corrections appeared in 1990. A Celtic miscellany (1951), which achieved wide distribution as a Penguin Classics paperback, was a collection of his limpid translations from the literature of all six Celtic languages; in the commentary he criticised the nineteenth-century and ‘Celtic twilight’ view of Celtic literature as particularly mystical or sentimental – Jackson was inclined to think classical Celtic literature rather displayed the hard-edged and unreflective mentality of adolescence.
In Language and history in early Britain (1953; republished several times and in print more than fifty years after its first appearance) Jackson traced the development of the P-Celtic languages (Breton, Welsh, and Cornish) from Roman times to the twelfth century. He hoped to produce a matching study of the Q-Celtic languages, Irish, Manx, and Scots Gaelic; although he never completed this project, in 1955 he produced Contributions to the study of Manx phonology (1955). A historical phonology of Breton (1967) was the first such exhaustive study undertaken for any Celtic language. He also published two short studies, The Gododdin: the earliest Scottish poem (1969; it is written in what is now called Welsh), and The Gaelic notes in the Book of Deer (1972). The international popular tale and early Welsh tradition (1961) favours the view that King Arthur was a historical rather than a purely mythological figure. The oldest Irish tradition: a window on the iron age (1964) argues that the Ulster sagas can provide genuine insights into a society much older than the composition and transmission of the existing texts.
Jackson's edition of Aislinge Meic Con Glinne was begun in 1955; although he greatly wished to prepare a student edition of this text because of its readability and Joycean qualities, he was able to concentrate on it only after his retirement in 1979 and his work was interrupted by a stroke in 1984. Fellow scholars assisted him in preparing it for the press, and when it appeared in 1990 the extensive glossary and concordance, intended to provide a broad sketch of the morphology of Middle Irish, incorporated material from his projected Historical grammar of Irish, abandoned because of his increasing ill health.
Jackson was a fellow of the British Academy (1957) and a member of the RIA (1965), and received a CBE in 1985; he also held numerous honorary degrees. On 12 August 1936 he married Janet Dall Galloway, a social worker from a farming family in Kinross, Scotland; they had a son and a daughter. He died 20 February 1991 at Edinburgh. He is regarded as one of the most brilliant Celticists of his time; his erudition commands the awe of a more specialised generation and even those who criticise him on particular points regard his work as foundational.