Seymour, St John Drelincourt (1880–1950), clergyman and historian, was the youngest son of Elizabeth Seymour (née Topp), and of the Rev. William F. Seymour, Church of Ireland clergyman in the united dioceses of Cashel and Emly, and was born 15 April 1880 at Abington rectory, Co. Limerick. He was educated at Tipperary Grammar School and entered TCD in 1898 as a pensioner. He graduated BD and was ordained in the Church of Ireland. In 1906 he was instituted into the Loughmoe Union in Cashel and Emly and then (1908) into the Toem Union, which included Donohill, from which he subsequently dated his writings. He subsequently rose to be archdeacon of Cashel and Emly. On 8 October 1903 he married Mary Seymour Strich, fourth daughter of John R. Strich of Dublin.
Seymour combined his ministerial duties with scholarship. Regarding local histories as the prerequisite for any comprehensive account of the entire country, in 1908 he published a succession list of the clergy of the dioceses of Cashel and Emly. It was followed in 1913 by The diocese of Emly. The trilogy was completed with the publication in 1930 of an account of the church plate and parish records of Emly and Cashel. Quickly Seymour's scholarly interests widened. Rushed forays from the countryside into the Dublin archives may have alerted him to promising materials. In 1900 he reedited and published the autobiography of Devereux Spratt (1620–88), a seventeenth-century clergyman. He appreciated the potential of the depositions of protestants’ losses in and after 1641, housed in TCD, and the wills and official records of the interregnum located in the Dublin PRO. He used the latter sources to excellent effect in The puritans in Ireland (1921) (reissued 1969), which traced, explained, and evaluated the ecclesiastical policies of the Cromwellian era. Meanwhile Seymour displayed greater intellectual adventurousness in his Irish witchcraft and demonology (1913). Despite the potential sensationalism of title and subject, his handling was firmly anchored in the evidence. Moreover, he compared Ireland, with its seeming rarity of formal witchcraft accusations and prosecutions, with the different situations elsewhere in Europe and North America. Seymour read the latest literature from France and America. He was feeling his way towards an explanation of witchcraft as culturally as well as socially and economically determined. Pioneering and innovative, the study remains the only sustained one. His treatment looked forward to the way in which the subject was later handled. True Irish ghost stories (1913) (compiled with H. L. Neligan and reissued 1926, 1990, and 2005) grew out of the earlier book. Cryptically Seymour commented that the intolerant attitude currently common ‘will soon become out of date’.
A wish to investigate the origins of the cultural world of which he was a part, but which during his life was under threat, led to other researches, particularly into the literary achievements of the Anglo-Norman settlers in Ireland. A series of essays culminated in Anglo-Irish literature, 1200–1582 (1929). The range and quality of his publications earned him a doctorate of letters from his old university and in 1922 election to membership of the RIA. Respected by his seniors, he was regarded as one of the most reliable and versatile of the Church of Ireland's historians. As such he was deeply involved in the collaborative project to publish in 1932 a history of the Church of Ireland under the editorship of W. A. Phillips (qv). The volumes were timed to coincide with the anniversary of St Patrick's arrival in Ireland and the catholics’ more flamboyant Eucharistic Congress. Seymour undertook several of the medieval chapters. Thereafter, he published virtually nothing. He remained archdeacon of Cashel until a month before his death on 25 May 1950. His widow and two sons survived him. Intellectual probity, intimate knowledge of sources (unpublished and published), and independence of judgement enabled him to evolve from a clerical antiquary into a church and literary historian of impressive range and achievements. In particular, his accounts of the Cromwellian church and witchcraft retain permanent value. Some of his research notes, often from documents destroyed in 1922, eventually passed to the library of the Representative Church Body in Dublin, where they have been used by later investigators.