Cosby, Pole (1703–66), ‘improving’ landowner, was born 14 April 1703, the only son of Dudley Cosby, landowner, and Sarah Cosby (née Pole), of Stradbally Hall, Queen's Co. (Laois). The Cosbys were among the first planter families in Queen's Co. in the 1550s, and became one of the key protestant landowning families in the seventeenth and eighteenth centuries. Pole was educated in the household of his uncle William Pole at nearby Ballyfin House till the age of 6. In 1709 he learnt French at a school in Portarlington before attending schools in York. In 1716 he returned to Ireland and attended Athy school. In 1721 he entered the University of Leiden, where he studied classics. Between June and September 1723 he toured the Low Countries with Warner Westenra, a friend from Queen's Co. After finishing his studies on the Continent he stayed in London briefly before returning to Ireland in April 1724 in time for his twenty-first birthday.
From 1724 he managed the Queen's Co. estate jointly with his father, and his marriage in 1727 brought with it additional landed income. When his father died (1729), he inherited the entire estate, which comprised about 7,000 acres (2,800 ha) and an annual income of about £2,000. Cosby was unable to secure any of the key county offices. He made an unsuccessful attempt (at a cost of at least £300) to hold on to his father's seat in the Dublin parliament as knight of the shire for the county in 1729. Though the rental on his estate increased rapidly in the 1720s and 1730s, Cosby spent far beyond his means and soon ran into serious debt. He and his family lived in a modest town house in Clifton, near Bristol, during 1733–4 in order to economise. But his exposure to the new standards of hospitality and display found in English households led him to invest even more capital on improvement schemes at Stradbally. His autobiographical account for the period 1703–c.1740 – one of the most valuable records of protestant landed society – outlines his achievements in developing the town of Stradbally, repairing churches, building bridges, remodelling his seat, and planting trees. In 1742 Cosby's head gardener, Daniel Collins, won the Dublin Society's £10 premium for planting timber trees: in 1741 alone, 13,835 trees were planted on the Stradbally demesne. In 1738 Cosby established a charter school at Stradbally for twenty boys and twenty girls. By 1740 he had run up a debt of at least £6,000, three times his annual income. Cosby did not cut back on expenditure: he rented town houses periodically in Dublin for the season, bought clothes and pictures of the newest fashion, and lived in a style similar to that of the very richest country esquires elsewhere in Ireland.
The last three decades of his life are not documented, but it is clear that his over-expenditure and the resultant indebtedness caused great distress to his children. In 1760 his eldest son threatened to take legal action against him for whittling away his inheritance. His children even advised him to fly to Holland because his ‘life was not safe in the three kingdoms’ (Cosby papers, T3829/H/4). It is possible that both Pole Cosby and his son suffered from mental illness. The latter was described as ‘insane’ in diplomatic correspondence in 1765.
Pole Cosby married (1727) Mary, daughter of Henry Dodwell; they had at least one son and three daughters. Their eldest son, Dudley Alexander Sydney Cosby, served as minister plenipotentiary to the court of Denmark (1765) and was made Lord Sydney of Leix and Baron of Stradbally (1768); he died childless in 1774 and the title became extinct. Pole Cosby died in 1766. His portrait was painted c.1743 by James Latham (qv) (reproduced in Crookshank), and by an unknown artist (reproduced in the Kildare Archaeological Society Journal).