O'Brien, Donough (d. 1624), 4th earl of Thomond , nobleman and president of Munster, was the eldest son of Conor O'Brien (qv), 3rd earl of Thomond, and his second wife, Úna, daughter of Turlough O'Brien Arra (known as Mac I Brien Arra). His date of birth is unknown, but his parents’ marriage took place after mid 1560, when his father's first wife died. Donough O'Brien had two brothers, Tadhg of Dromore (d. c.1642), who was a supporter of Hugh O'Donnell (qv) during the Nine Years War and a political opponent of the earl, and Daniel or Donal (qv), who was an MP in the 1613–15 and 1634 parliaments and was created 1st Viscount Clare by Charles II in 1662. His three sisters married men of moderate political influence: Mary married Turlough Roe MacMahon of Clonderalaw, and was mother to Máire Rua O'Brien (qv) of Leamaneh; Honora married Thomas Fitzmaurice (qv), 16th Lord Kerry and Baron Lixnaw; and Margaret married James Butler, 2nd Viscount Dunboyne.
Donough O'Brien was brought up as a protestant at the court of Queen Elizabeth. He was there in 1577, when his father, Conor, obtained a new patent from the queen and Donough was created baron of Ibrackan, a move perhaps designed to signal his right of succession to the earldom. He remained a loyal subject and committed protestant throughout his life. Sir Nicholas Malby (qv), president of Connacht, commented that Donough was ‘glad when he may have the company of any English gentleman’ (TNA, SP 63/103, no. 41). He succeeded to the title of earl of Thomond on the death of his father early in 1581, returned to Ireland the following year, and had livery of his father's lands on 15 July 1582. He was a member of the parliament convened in spring 1585 in Dublin, where he became involved in a dispute over precedence with Ulick Burke (qv), 3rd earl of Clanricard. Thomond was an active participant in the negotiation of the 1585 composition of Connacht, under which Gaelic exactions were commuted to fixed annual payments. De facto recognition was given to the earl as overlord, since his own demesne lands were exempted from composition rent and he was awarded payments of 5s. per quarter of land in seven of the nine baronies in Co. Clare, the exceptions being the barony of Corcomroe in which the jurisdiction of his father's great rival Sir Donal O'Brien was recognised, and the barony of Inchiquin, where the overlordship of Murrough O'Brien was recognised. In 1588 the 4th earl of Thomond became a member of the provincial council for Connacht (which then had jurisdiction over Co. Clare) and he regularly provided support to the governor, Sir Richard Bingham (qv), in his military exploits in north Connacht. In December 1589 the queen awarded him an annual pension of £200 for life.
During the Nine Years War (1594–1603) Thomond supported the military campaigns of the Dublin administration; his home territory was sometimes a target for rebellious activity in his absence, most notably in 1599 and 1600. He was away in London in 1598 at the time of the Ulster lords’ victory over English forces at the battle of the Yellow Ford in Ulster. In the following year, he was part of an ill-fated expedition by the earl of Essex (qv) in Munster. Early in 1600 Thomond allied himself with the newly appointed Munster president, Sir George Carew (qv), who also had the active support of Thomas Butler (qv), 10th earl of Ormond. Thomond's actions saved Carew from being taken prisoner by O'More rebels in April 1600 and Carew showed him special favour thereafter. Thomond was at the royal court again in 1601, returning home just in time to assist in the English defeat of Spanish and Irish forces at the battle of Kinsale. At court Thomond was disliked by Sir Robert Cecil, secretary of state, who delayed a decision on a request to have Co. Clare removed from Connacht. Despite this, in November 1602 Lord Deputy Mountjoy (qv) removed Clare from the jurisdiction of the Connacht president and Thomond himself was appointed governor of the county. This arrangement was confirmed in September 1603 and Thomond was also appointed to the Irish privy council.
Donough O'Brien's upbringing at court and his subsequent career in Ireland made him a virtual stranger in his own earldom. Although he attempted to cultivate popularity at home through his patronage of professional poets, some of the extant political poetry about him is at best ambivalent and occasionally satirical. His favour with the English administration was at the price of his popularity at home and in 1608 he was supplied with English horsemen and footmen for his protection. Writing in his support in 1602 Sir George Carew commented ‘he is the first and last of Ireland birth that ever I found wholly addicted to the queen as he is. For her sake he is hated of all his nation’ (TNA, SP 63/211, no. 37). The earl was considered for the post of president of Munster when a vacancy arose in 1599 and again in 1607. Eventually, in March 1615, he was appointed president of the province, an office he held until his death in 1624 after which the post reverted to an Englishman, Sir Edward Villiers (d. 1626), half-brother to the influential duke of Buckingham. In March 1616 he resigned the governorship of Clare in favour of his son Henry O'Brien (qv), thereby ensuring that the county continued to be exempt from the jurisdiction of the provincial presidency.
The earl's demesne lands were in the south of Co. Clare, in the vicinity of his principal house at Bunratty. He moved to the well-fortified castle at Bunratty from Clonroad in the mid 1580s. He devoted much effort to securing title under English common law to lands in Thomond and to developing an English-style landed estate there. He planted English tenants in the vicinity of Bunratty and at Sixmilebridge, as well as on his estate in Co. Carlow. He introduced Dutch tenants to settlements in west Clare. He also owned former church land in Co. Dublin and extended his landed interests into Tipperary and Limerick, assisted by the well-known speculators Sir George Carew and Sir Richard Boyle (qv), earl of Cork. Late in his career he enjoyed the support of Sir Charles Coote, Roger Jones (qv), and Lord Deputy Falkland (qv), and was thus confident that Thomond would be exempted from any future government-sponsored plantation initiative.
Donough O'Brien first married Ellen (Eveleen) Roche (d. 1583), daughter of Maurice Roche, Viscount Fermoy; his second wife was Lady Elizabeth Fitzgerald, daughter of Gerald Fitzgerald (qv), 11th earl of Kildare. There were two sons from the second marriage, Henry, who succeeded as 5th earl in 1624, and Barnabas (qv), who succeeded as 6th earl in 1639. The 4th earl of Thomond died at Clonmel on 5 September 1624, having been predeceased by his wife Elizabeth on 12 January 1618. He died a wealthy man. In his will, dated 28 November 1617, preserved in Petworth House archives, he bequeathed his earldom to his son Henry, ‘whose rightful inheritance it is’, and made provision for many other family members and political and ecclesiastical associates while scarcely mentioning his second son, ‘Bryan’ (Barnabas). Henry, 5th earl of Thomond, duly erected an elaborate monument to the memory of his father in St Mary's cathedral, Limerick, the burial place having been selected by Donough himself. A portrait of the 4th earl survives at Dromoland castle.