|Intro||Scottish peeress "suo jure"|
|Birth||5 January 1926|
|Death||16 May 1978 (Oban)|
Diana Denyse Hay, 23rd Countess of Erroll (5 January 1926 – 16 May 1978) was a Scottish noblewoman.
Diana was the only child of Josslyn, Earl of Errol and his first wife Lady Idina Sackville. When her father was murdered in 1941, she inherited the earldom of Erroll and the lordship of Hay, while the barony of Kilmarnock, which could only be inherited by a male heir, passed to her uncle Gilbert Boyd. She also inherited the hereditary position Lord High Constable of Scotland.
On 19 December 1946, at St Margaret's, Westminster, Lady Erroll married Sir Iain Moncreiffe of that Ilk, the Chief of Clan Moncreiffe and a herald at the Court of the Lord Lyon. They had three children:
- Merlin Sereld Victor Gilbert, later 24th Earl of Erroll (b. 20 April 1948), 4 children.
- Hon. Peregrine David Euan Malcolm Moncreiffe of that Ilk, Chief of Clan Moncreiffe (b. 16 February 1951), married in 1988 Miranda Mary Fox-Pitt (b. 29 December 1968), 6 children.
- Lady Alexandra Victoria Caroline Anne Moncreiffe Hay (b. Edinburgh, 30 July 1955), married Jolyon Connell (b. Edinburgh, 16 May 1952) and had Flora Diana Katharine Cecilia Connell (b. London, 24 February 1990); Ciara Edith Elisabeth Connell (b. London, 28 March 1994); she also has a son, Ivar Francis Grey de Miremont Wigan (b. Perth, 25 March 1979).
After the passing of the Peerage Act 1963 which allowed suo jure peeresses to take a seat in the House of Lords, Lady Erroll did so, with 11 other peeresses. Moncreiffe and Lady Erroll were divorced in 1964 and on 27 November that year, she married, in Lonmay, Major Raymond Carnegie (born 9 July 1920, a grandson of Charles, Earl of Southesk) and they had one son, the Hon. Jocelyn Jacek Alexander Bannerman Carnegie (b. 21 November 1966), who married Susan Mhairie Butler in 1990; the couple had six children.
Lady Erroll died in 1978, aged 52. Her titles passed to her eldest son, Merlin. The cause of death has never been publicly disclosed.