O'Brien, Conor (1535–81), 3rd earl of Thomond , was the eldest of three sons of Donough O'Brien (c.1515–1553), 2nd earl of Thomond, and Ellen (Helen; d. 2 July 1597), youngest daughter of Piers Butler (qv), 8th earl of Ormond and 1st earl of Ossory. None of the brothers reached old age: Turlough died in the same year as Conor, 1581, while the youngest brother had died in 1567. As he was only seventeen, Conor O'Brien did not succeed to the earldom of Thomond on the death of his father in February 1553; instead, control of the earldom passed to his father's younger brother, Sir Donal O'Brien, some twenty years Conor's senior. Sir Donal enjoyed local support as lord of the O'Briens and successfully asserted his claim to the earldom for much of the reign of Queen Mary (1553–8).
Following the arrival of Sir Thomas Radcliffe (qv), earl of Sussex, as lord deputy of Ireland in spring 1558, Conor O'Brien was restored to the earldom and his uncle proclaimed a traitor. In Limerick cathedral on 10 July 1558 Conor swore loyalty to the English monarch and was installed as earl. A poem by Donal Mac Bruaideadha describes the display of military strength in the ceremonial procession at Magh Áine on the occasion of the installation, with the lord deputy and the earls of Clanricard (qv), Ormond (qv), and Desmond (qv) in attendance (McKenna (ed.), poem 27). While the earl marked his new-found status through the patronage of poets, he did not generally find favour with them, and Uilliam Óg Mac an Bhaird accused Conor of executing three poets in 1572 (Ó Cuív). Two years later a Thomond poet, Maoilín Óg Mac Bruaideadha (qv), complained of his poverty because his patron, the earl, had abandoned him (O'Rahilly (ed.), I, poem 26).
Although Conor O'Brien enjoyed the title of earl after 1558, his uncle Sir Donal O'Brien continued to be an influential figure in the Thomond lordship, a fact recognised by the English administration, who appointed him sheriff of the county. The earl's political position remained insecure and his uncle successfully renewed his challenge for a period in 1563–4. Ultimately, the two rivals agreed a compromise: Conor was recognised as earl and Donal was compensated by being granted jurisdiction over the barony of Corcomroe in the north-west of Co. Clare, where his lands lay, and where he established himself in the castle of Ennistymon. He later extended his area of control to include the adjoining barony of Burren. Conor O'Brien's principal strongholds were in the south of the county, at Bunratty, Clare castle, and Clonroad, where his father had built an English-style manor house similar to the Butler house at Carrick-on-Suir, Co. Tipperary. The structure at Clonroad, near Ennis, had some defensive features, being accessed by a bridge over the River Fergus and surrounded by a curtain wall. The house itself, as depicted by Thomas Dingley (qv) in the 1680s, was a six-bay, two-storey building with large mullioned windows, adjoining an earlier tower house.
On the appointment of Sir Edward Fitton (qv) as first president of Connacht in 1569, with jurisdiction over Thomond (newly designated as Co. Clare), his authority was initially opposed by the earl, who fled to France rather than submit to the president. However, on 1 November 1570 Thomond submitted to Queen Elizabeth at court and he returned to Ireland before the end of the year, agreeing to keep the peace and serve the queen faithfully. Thomond enjoyed the support of his influential cousin, Thomas Butler, 10th earl of Ormond, and he in turn generally supported Ormond and the English monarch. His relationship with successive Connacht presidents remained guarded, and when Sir Nicholas Malby (qv) introduced the first ‘composition’ agreement in 1577, designed to help finance the Connacht provincial administration, the earl of Thomond called for the abolition of the presidency.
Conor O'Brien married firstly, in 1559, Eveleen, widow of James Fitzgerald (qv), 13th earl of Desmond, daughter of Donal MacCarthy, but she died the following year. He subsequently married Úna, daughter of Turlough O'Brien Arra, of Co. Tipperary, with whom he had three sons, Donough O'Brien (qv), 4th earl of Thomond, Tadhg O'Brien of Dromore (d. c.1642), and Daniel (Donal) O'Brien (qv), 1st Viscount Clare, and three daughters, Mary, Honora, and Margaret. He arranged for his eldest son, Donough, to be brought up as a protestant and educated at the Elizabethan court in preparation for his role as a future earl of Thomond. Conor O'Brien himself remained a catholic. He died ‘in the very prime of his life’ (AFM, s.a. 1580) at the age of forty-five, early in 1581, and was buried at the Franciscan friary at Ennis, the traditional burial place of his family. His life-long rival, Sir Donal, had been buried there some eighteen months earlier. Conor O'Brien was survived by his mother, Ellen, and by his widow, Úna, who died at Clare castle, Co. Clare, in 1589.