Sarah Hussey Delaval was the sixth and youngest daughter and co-heir of John Hussey Delaval, later Baron Delaval of Seaton Delaval and his first wife, Susannah (d. 1783). On 3 June 1780, she married George, 2nd Earl of Tyrconnel as his second wife. They had one child, Lady Susannah Carpenter, who was heiress to the Earl. The Countess of Tyrconnel left her husband to live with the Earl of Strathmore but died on 7 October 1800 at Gibside, Co. Durham. She was buried in Westminster Abbey on 4 November 1800, aged 37.
Downman painted oil portraits of various members of the Delaval family, notably Lady Delaval (G. C. Williamson, John Downman, London, 1907, p. 13) and Miss Delaval (p. 52). There is a miniature of Lady Delaval dated 1792in the Victoria and Albert Museum (Salting Bequest P. 87-1910) which is a replica of an oil painting in the National Gallery. Two other miniatures of the Countess of Tyrconnel by Downman were sold Christie's, 24 November 1981, lot 32 and 27 November 1989, lot 160. A portrait of the sitter's father, also by Downman, was sold Christie's, 30 April 1996, lot 143.