O'Donnell, Mary Stuart (d. p.1639), heiress, was born in England c.1607, the daughter of Ruaidhrí O'Donnell (qv), earl of Tyrconnell, and Bridget Fitzgerald, daughter of Henry Fitzgerald (qv), 12th earl of Kildare, and Frances Howard. Her life was largely shaped by the decision of her father to flee Ireland prior to her birth. Tyrconnell did not have enough time to bring his pregnant wife with him on his flight, but appears to have assumed that she would eventually follow him. However, Bridget was furious with her husband for apparently abandoning her and instead travelled to the royal court at London to deny all knowledge of her husband's intentions. With the help of her grandfather Charles Howard, earl of Nottingham and lord high admiral of England, Bridget charmed King James I, who granted her a pension of £200 a year from the revenues of her husband's estates. The king also had Bridget's newly born daughter styled as Mary Stuart, after his own mother. In 1609 Bridget returned to her family's estates in Kildare and raised Mary a catholic. Mary returned to England in 1619 to live with the Howards, and received a pension and a generous dowry from the king.
However, the pension was often not paid on time and Mary struggled financially. To the consternation of her mother and grandparents she started associating with young and disaffected Irish catholics based in London, and began a relationship with Dudley (Dualtach) O'Gallagher. The Howards attempted to arrange her marriage to an English protestant, but she refused. Then, in the summer of 1626, Mary and some friends forced their way into a London jail and freed Caffar O'Donnell (qv) and Hugh O'Rourke. Following an investigation, she was summoned to appear before the royal council, but instead went into hiding, before escaping from England in disguise with O'Gallagher at the end of 1626.
Her arrival in Brussels in January 1627 caused a great stir and she was brought before the Infanta Isabella, who granted her a pension. On 13 February 1627 Pope Urban VIII wrote to Mary, praising her steadfastness to the catholic faith. Meanwhile, Florence Conry (qv), archbishop of Tuam, arranged for her marriage to John O'Neill, earl of Tyrone. Relations between the O'Neills and the O'Donnells were strained at this time, and Conry hoped that this union would create harmony between the two families. However, Mary, who had kept her relationship with O'Gallagher secret, refused once again to be forced into a marriage, much to the annoyance of her compatriots. Discontented, she opened up secret communications with English officials for her possible return to England, claiming that she could draw her brother Hugh, earl of Tyrconnell, back into the king's service. Ultimately, nothing came of this.
About 1629–30, her relationship with O'Gallagher was exposed when she became pregnant. The couple fled Brussels in disgrace and were married at Rome. With no source of income, they unsuccessfully petitioned the pope for maintenance, before settling in Genoa and then Vienna, where the Irish Franciscans treated them kindly. However, they were shunned by most of the Irish exiles and Tyrconnell claimed that Mary was an imposter. O'Gallagher became a captain in the imperial army in 1635, but was killed soon afterwards. In 1639 Mary was living at Rome, married to a poor Irish captain after which nothing more is known of her life.