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Mary O'Brien, 3rd Countess of Orkney (c. 1721 – 1790) was the eldest daughter of Anne O'Brien, 2nd Countess of Orkney, and William O'Brien, 4th Earl of Inchiquin, and Countess of Orkney in her own right.

She was deaf and was married by signs, in 1753, to her first cousin, Murrough O'Brien, fifth Earl of Inchiquin, first Marquess of Thomond, and first Baron Thomond, of Taplow, in England, K.P. She lived with her husband at his seat, Rostellan, near Cork Harbour. She succeeded to the Earldom on 6 December 1756,^[1] when her mother died without male issue. By his Lordship, she had an only surviving daughter, Mary, the fourth Countess. The Marquess of Thomond died in consequence of his horse falling with him in Grosvenor Square, on 10 February 1808, in his 85th year. The Countess of Orkney died on 10 May 1790.

References

^ Cokayne, G. E.; H. A. Doubleday & Lord Howard de Walden, eds. (1940). The Complete Peerage, or a history of the House of Lords and all its members from the earliest times, volume XIII: Peers created 1901 to 1938. 13 (2nd ed.). London: The St. Catherine Press. Volume X, p. 109.

Peerage of Scotland
Preceded by Anne Douglas-Hamilton	Countess of Orkney 1756–1791	Succeeded by Mary FitzMaurice

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[1] Cokayne, G. E.; H. A. Doubleday & Lord Howard de Walden, eds. (1940). The Complete Peerage, or a history of the House of Lords and all its members from the earliest times, volume XIII: Peers created 1901 to 1938. 13 (2nd ed.). London: The St. Catherine Press. Volume X, p. 109.

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