Threlkeld, Caleb (1676–1728), doctor, minister of religion, and botanist, was born 31 May 1676 in Keybergh or Keibergh in the parish of Kirkoswald, Cumberland, England, third among three sons of Thomas Threlkeld and Bridget Threlkeld (née Brown), who both died in 1712. He also had three younger sisters. He matriculated at Glasgow University (1696), but may not have graduated; however, by 1699 he seems to have become a minister in Low Huddlesceugh near Kirkoswald, probably associated with a dissenting congregation there. Threlkeld also practised as a doctor, and sought a medical degree from the University of Edinburgh, granted in January 1713. Two months later he moved to Dublin, leaving his family to follow a year later; he appears at first to have preached at a conventicle in the city, but prospered as a doctor.
He had been interested in botany since his student days, and after twelve years collecting specimens in the vicinity of Dublin, published a list of around 530 plant names as Synopsis stirpium Hibernicarum (1726). This is the first botanical work of importance published in Ireland, and only the second to contain original observations on Irish botany. It contains an appendix of previously unpublished observations supplied by Thomas Molyneux (qv). Threlkeld also drew on an unpublished manuscript on indigenous plants compiled by the Rev. Richard Heaton (d. 1666), which probably provided him with over 400 names of native plants in the Irish language. This in itself is a novel aspect of Threlkeld's work, and he also translated from Latin some passages from earlier works, which he hoped would thus be of more benefit to less learned readers. Scholars, including some of Threlkeld's contemporaries, criticised the work as insufficiently scientific and too much derived from earlier authorities, notably John Ray's Synopsis methodica stirpium Britannicarum (1696), but recent research supports Threlkeld's own claim that his catalogue was the ‘first of its kind in the kingdom’ (quoted in Nelson (1979)), and he seems to have made his own collection of plants. Several hundreds of the plants he listed had not been published for Ireland before. He was sufficiently learned and sufficiently confident of his learning to challenge problems of nomenclature, and he recorded a good deal of interesting historical material. He was the first botanist to note some of the customs and beliefs associated with what the Irish called the seam rog or shamrock, and also the first to link these traditions with one botanical species. The work was issued three times in 1726 and 1727. Threlkeld died 28 April 1728 in Dublin, of a violent fever, and was buried in the burying ground of St Patrick's cathedral, Dublin.
He married (7 March 1698), in Edinburgh, Elizabeth, daughter of Alexander and Jennet Dalrymple of Auchin Houry, near Glasgow. He and his wife had three sons and four daughters, of whom at least one daughter died young. He was greatly regretted by the poor people to whom he had been a kind benefactor. A botanical genus was named Threlkeldia in his honour; fragments of what was probably his herbarium survive in TCD.