Donough was brought up at Elizabeth's court. There he was residing in 1577, when he was mentioned as Baron of Ibrickan in the new patent granted on 7 Oct. to his father. On his father's death in 1581 he succeeded him as fourth earl of Thomond; by 1582 he had returned to Ireland, and, though some suspicion seems to have been entertained of his loyalty, he was assiduous in his attendance upon the lord-deputy in 1583 and 1584. His main object was to obtain an acknowledgment that the county of Clare, where his possessions were situated, was part of Munster, and thus to free it from the jurisdiction of the Connaught government, under which it had been placed previous to his father's death (Bagwell, Ireland under the Tudors, iii. 127); but it was many years before he succeeded. In 1584 he was one of the commissioners who established the agreement that tanistry and the law of partible succession should be abolished in Connaught, and a tax of ten shillings a quarter be paid on land. Next year he attended the parliament held at Dublin in April. In 1589 he was active in subduing the rebellious Irishry in the mountains; and when Tyrone's rebellion broke out in 1595, he played a considerable part in its suppression. In command of a large force, he passed the Erne in July and invaded O'Donnell's country, but retreated in August when a truce was signed. In the following September he was detached by Sir William Russell [q. v.], with five companies of foot and 145 horse, for the defence of Newry. In 1597 he served in Lord Burgh's campaign, but early next year proceeded to England, arriving in London on 19 Jan. 1598; there he remained during the greater part of the year, and produced a favourable impression.
Meanwhile Tyrone's victory at the Yellow Ford was followed by the spread of disaffection into Thomond's country. Teige O'Brien, Thomond's next brother, entered into communication with Tyrone's son, and joined the rebels. In 1599 O'Donnell invaded Clare, ravaging the country, capturing most of the castles, and making a prisoner of Thomond's youngest brother, Daniel O'Brien [q. v.], afterwards first Viscount Clare, who had been left to defend it. Thomond returned from England, and after spending three months with his kinsman, the Earl of Ormonde, in collecting forces, he invaded Clare to revenge his brother's imprisonment and recover his possessions. He procured ordnance from Limerick, and laid siege to such castles as resisted, capturing them after a few days' fighting; at Dunbeg, which surrendered immediately, he hanged the garrison in couples on trees. The invaders were completely driven out of Clare and the neighbouring country, and the loyalists had their strongholds restored to them. During the rest of 1599 Thomond accompanied Essex on his progress through Munster, but left him at Dungarvan and returned to Limerick, being appointed governor of Clare on 15 Aug., and made a member of the privy council on 22 Sept.
During 1600 Thomond was constantly occupied in the war. In April he was with Sir George Carew, and narrowly escaped capture with the Earl of Ormonde; his prompt and vigorous action saved Carew's life and enabled them both to cut their way through their enemies, though Thomond was wounded (Stafford, Pacata Hibernia). He was present at an encounter with Florence MacCarthy Reagh [q. v.], and assisted at his submission in May. In June he was commanding in Clare and opposing O'Donnell's raids. He entertained the lord-deputy at Bunratty and marched out to oppose Tyrone's progress southwards, but no battle was fought, and Tyrone returned without having even seen an enemy. Next year, after holding an assize at Limerick in February, at which sixteen men were hanged, Thomond again went to England, probably with the object of obtaining the governorship of Connaught and of securing the union of Clare with Munster. He delayed there longer than was desired, and his return with reinforcements was eagerly looked forward to by the besiegers at Kinsale. At length he set out by Bristol, and, landing at Castlehaven on 11 Nov. 1601, proceeded to Kinsale, where he took a prominent part in the siege. After the surrender of Kinsale he proceeded through Munster, established himself in Bere Island, and was in command at the siege of Dunboy, and hanged fifty-eight of the survivors.
Till June 1602 he was constantly with the army. He then again visited England, and, as a recompense for his services, his request for the transfer of Clare was granted, though the lord-deputy and privy council of Ireland were opposed to the measure. He returned in October. As a further reward the queen ordered that his name should be always placed next to those of the lord-deputy and chief-justice in commissions of oyer and terminer and gaol delivery. On 30 July 1604 he was appointed constable of Carlow, and on 6 May 1605 he became president of Munster. In 1613 he strongly upheld the protestant party in opposition to the recusants in the disputes about the speaker of the Irish House of Commons; and on 17 May 1619 he was reappointed governor of Clare. He became one of the
