The arms are those of O'Brien with those of Sinclair on an escutcheon of pretence, as borne by Murrough, Earl of Inchiquin, born in 1726, who succeeded to the Earldom of Thomond in 1777. He married in 1753 his first cousin, Mary Sinclair, suo jure Countess of Orkney, who died in 1790. He was created, in 1800, Marquess of Thomond and died at his house in Grosvenor Square at the age of 80 in 1808 in consequence of a fall from his horse. His obituary in the entleman's Magazine says "[He] was celebrated rather as a bon vivant than a fashionable, and for many years had the reputation of being a six-bottled man."