Murrough O'Brien succeeded his uncle and father-in-law William as 5th Earl of Inchiquin, of Ireland, upon the latter's death 18 July 1777. On 29 December 1800 he was created 1st Marquess of Thomond, in the peerage of Ireland, and on 2 October 1801 became Peer of the United Kingdom as Baron Thomond. He married firstly 5 March 1753 to his cousin Mary (d. 10 May 1791), daughter of William 4th Earl of Inchiquin, with whom he had several children of which only one, Mary, survived. Inchiquin married secondly 25 July 1792 to Mary Palmer (d. 7 September 1820), daughter of John Palmer of Devon and neice and heiress to the painter Sir Joshua Reynolds. Inchiquin died on 8 February 1808 as the consequence of a fall from his horse in Grosvenor Square. With his death, the Baroncy of Thomond became extinct, but his Irish titles descended to his nephew, William, 2cd Marquess of Thomond and 6th Earl of Inchiquin (d. 21 August 1846).