She became Countess of Inchiquin, through her marriage to her first cousin William O'Brien, 4th Earl of Inchiquin,[1]the O'Brien, Chief of the Name, Prince of Thomond, and Lord of Dál gCais. They were married on 29 March 1720.[2] Their daughter, Lady Mary (c. 1721–1790), succeeded her mother in the earldom of Orkney.