O'Carroll (Ní Chearbhaill), Mairghréag (d. 1451), lady of Uí Failghe (Offaly), was daughter of Tadhg Ailbhe O'Carroll (qv) (d. 1407), lord of Éile (Ely) – her mother was probably Mór, daughter of Brian Sreamach O'Brien (qv) – and was a famous patron of the church and an aristocratic hostess during the first half of the fifteenth century. Mairghréag an einigh (the hospitable) was one of the recorded five children resulting from Tadhg's probable three marriages. Virtually nothing is known of her early life. There is a suggestion that she was betrothed from birth to An Calbhach O'Connor (qv) (d. 1458), son of Murchadh O'Connor Faly (qv) (d. 1421), lord of Uí Failghe, and perhaps of his recorded wife Gormflaith MacMurrough. The first documentary mention of Mairghréag confirms that she married An Calbhach before 1417. On that occasion she was mentioned among the complaints of John Talbot (qv), lieutenant of Ireland, against James Butler (qv), 4th earl of Ormond. Talbot alleged that Ormond and his brother Prior Thomas Butler (qv) of Kilmainham arranged for their lordship of Oghtryn, Co. Kildare, to pay a black rent to the wife of An Calbhach in 1417. Consequently, this arrangement allowed the O'Connor Falys to pass through Oghtryn and plunder the Pale, thus disrupting Talbot's government.
In the early fifteenth century the O'Connor Faly lordship expanded through the military ability of both Murchadh and An Calbhach. On Murchadh's death (1421) his brother Diarmaid succeeded him as lord of Offaly. His kingship was ineffective, as An Calbhach was recorded as king of Offaly in 1422. Through his military talent and political acumen, An Calbhach was one of the most powerful Irish leaders by 1430. His renown was doubly assured by his wife's banquets. Despite the famine of 1433, Mairghréag held two extravagant banquets at Killeigh and Rathangan. In Gaelic Ireland it was customary for the aristocracy to extend invitations to the learned classes to feast with them during religious holidays. On 26 March 1433 2,700 scholars, both Irish and Scottish, along with many poor men, attended a feast at Killeigh. Before the festivities, Mairghréag presented two golden chalices to the church. After the feast, she adopted two orphans for the good of her soul. The second banquet, held at Rathangan (15 August 1433), was both symbolic and illustrative of the territorial gains of An Calbhach. In 1432 Rathangan was in the hands of Gerald FitzGerald (qv), 5th earl of Kildare. Furthermore, the fact that it had been the pre-Norman caput of the O'Connor Faly dynasty undoubtedly added great significance to Mairghréag's banquet.
References to Mairghréag in bardic poems and annalistic entries reveal a deeply devout woman. The papacy granted An Calbhach and Mairghréag a plenary remission from sin in 1441. It has been suggested that both she and her husband may have been influenced by medieval Christian mysticism emanating from the Continent. Tentative evidence hints that she and An Calbhach may have taken vows of chastity towards the end of her life. In 1445 Mairghréag, with several other Gaelic Irish and Anglo-Irish nobles, travelled on pilgrimage to Santiago de Compostela in north-west Spain. There she received the indulgences of St James. Interestingly, during the nineteenth century female remains bearing a simple Spanish cross were discovered in the Franciscan friary of Killeigh, where Mairghréag was buried.
She returned safely to Ireland sometime in 1445, but several of her companions died in Spain. On her return, hostility erupted between An Calbhach and the de Berminghams of Carbury, Co. Kildare. MacGeoghegan, lord of Cénel Fiachach, his son, and the grandson of Art O'Melaghlin were captured by the English at parley in winter 1445. Without telling her husband, Mairghréag traded his hostages for the return of MacGeoghegan and the others. This act earned her the praise of several annalists. She died in 1451, apparently of breast cancer, and was interred at Killeigh. An annalist fittingly described her as the best woman of her time in Ireland. Mairghréag and An Calbhach had seven recorded children.