Mary O'Brien, 3rd Countess of Orkney Scottish noble (1721–1791) Parents William O'Brien, 4th Earl of Inchiquin Anne O'Brien, 2nd Countess of Orkney Children Mary FitzMaurice, 4th Countess of Orkney Siblings William O'Brien Augustus O'Brien George O'Brien Murrough O'Brien Partners Murrough O'Brien, 1st Marquess of Thomond