Mary O'Brien, 3rd Countess of Orkney (c. 1721–1791) was the eldest daughter of Anne O'Brien, 2nd Countess of Orkney

Anne O'Brien, 2nd Countess of Orkney

Anne Douglas-Hamilton, 2nd Countess of Orkney was a Scottish noblewoman and the eldest daughter of Field Marshal George Hamilton, 1st Earl of Orkney, and Elizabeth Villiers....

 and William O'Brien, 4th Earl of Inchiquin

William O'Brien, 4th Earl of Inchiquin

William McWilliam O'Brien, 4th Earl of Inchiquin, KB, PC was an Irish peer and politician.-Background:O'Brien was the eldest son of William O'Brien, 3rd Earl of Inchiquin and his wife, Mary , sister of the 1st Earl of Jersey, and inherited his father's titles in 1719.-Political career:Inchiquin...

, and Countess of Orkney

Earl of Orkney

The Earl of Orkney was originally a Norse jarl ruling Orkney, Shetland and parts of Caithness and Sutherland. The Earls were periodically subject to the kings of Norway for the Northern Isles, and later also to the kings of Alba for those parts of their territory in mainland Scotland . The Earl's...

 in her own right

Suo jure

Suo jure is a Latin phrase meaning "in her [or his] own right".It is commonly encountered in the context of titles of nobility, especially in cases where a wife may hold a title in her own right rather than through her marriage....

.

She was deaf and dumb

Deaf-mute

For "deafness", see hearing impairment. For "Deaf" as a cultural term, see Deaf culture. For "inability to speak", see muteness.Deaf-mute is a term which was used historically to identify a person who was both deaf and could not speak...

 and was married by signs

Sign language

A sign language is a language which, instead of acoustically conveyed sound patterns, uses visually transmitted sign patterns to convey meaning—simultaneously combining hand shapes, orientation and movement of the hands, arms or body, and facial expressions to fluidly express a speaker's...

, in 1753, to her first cousin, Murrough O'Brien

Murrough O'Brien, 1st Marquess of Thomond

Murrough O'Brien, 1st Marquess of Thomond KP, PC , known from 1777 to 1800 as the 5th Earl of Inchiquin, was an Irish peer, soldier and politician.-Life:Murrough O'Brien was born in 1726 to Hon...

, fifth Earl of Inchiquin, first Marquess of Thomond, and first Baron Thomond, of Taplow

Taplow

Taplow is a village and civil parish within South Bucks district in Buckinghamshire, England. It sits on the east bank of the River Thames facing Maidenhead on the opposite bank. Taplow railway station is situated near the A4 south of the village....

, in England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

, K.P.

Order of St. Patrick

The Most Illustrious Order of Saint Patrick is a British order of chivalry associated with Ireland. The Order was created in 1783 by George III. The regular creation of knights of Saint Patrick lasted until 1921, when most of Ireland became independent as the Irish Free State...

 She lived with her husband at his seat, Rostellan, on the harbor of Cork. She succeeded to the Earldom on the 5th of December, 1766, when her mother died without male issue. By his Lordship, she had an only surviving daughter, Mary

Mary FitzMaurice, 4th Countess of Orkney

Mary FitzMaurice, 4th Countess of Orkney was a Scottish suo jure peeress, the only surviving child of Murrough O'Brien, 1st Marquess of Thomond and Mary O'Brien, 3rd Countess of Orkney....

, the fourth Countess. The Marquess of Thomond died in consequence of his horse falling with him in Grosvenor Square, on the 10th of February, 1808, in the 85th year of his age. The Countess of Orkney died on the 10th of May, 1790.