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, on the harbor of Cork. She succeeded to the Earldom on 5 December 1766, 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 the 10th of May 1790.