O'Brien (Ó Briain), Toirdhealbhach Donn (d. 1528), king of Thomond, was son of Tadhg an Chomhaid O'Brien (qv), king of Thomond, and Annabella, daughter of Ulick fitz Ulick Burke. The career of Tadhg, who dramatically expanded O'Brien influence across the Shannon into Limerick and Tipperary in 1466, made a profound impression on his son. After Tadhg's death that year, he was succeeded as king of Thomond by his brother, Conchobhar O'Brien (qv). From the middle years of the 1470s, Conchobhar encountered opposition to his kingship, but it was the emergence of his nephew, Toirdhealbhach Donn, in the 1480s that really threatened his crown. No doubt Toirdhealbhach Donn's rise was faciliated by his marriage to Raghnailt, daughter of Seaán MacNamara, the important O'Brien vassal; and by 1486 Toirdhealbhach Donn was in control of much of east Thomond. However, the death of his wife that year saw him gradually begin to look for allies outside Thomond. Soon he allied himself to the FitzGeralds of Desmond, marrying Eleanor FitzGerald in 1490. Indeed, the power of Toirdhealbhach Donn largely reduced Conchobhar to the status of a figurehead.
After 1492 the O'Briens aligned themselves with their kinsman Sir James Butler (qv) of Ormond, Henry VII's governor, against the former lord deputy Gerald FitzGerald (qv), 8th earl of Kildare. During this period Conchobhar died (1496) and was succeeded by his aged brother Toirdhealbhach Óg O'Brien. It would seem that Toirdhealbhach Óg's accession had the approval of Toirdhealbhach Donn, and on the former's death (1498) Toirdhealbhach Donn was acclaimed by his dynasty as king of Thomond. First on his agenda was expansion into Limerick and Tipperary; secondly, he was eager to revenge the killing of Sir James Butler of Ormond by Sir Piers Butler (qv) on 17 July 1497. This impending O'Brien aggression alarmed Kildare. As a result he petitioned the king for troops to resist the O'Briens, as well as warning his son-in-law Sir Piers Butler. In spite of this warning, Toirdhealbhach Donn marched into the Ormond earldom, defeating the Butlers in 1499.
Toirdhealbhach Donn's enmity to the Kildares and their allies continued, and he made destructive raids throughout Limerick during winter 1501. Indeed, he cobbled together a confederation of Irish and Anglo-Irish lords to oppose the extension by Kildare of his own and Tudor power. On 19 August 1504 these opposing forces met at Knockdoe, near Galway, fighting a savage encounter resulting in the victory of Kildare. Toirdhealhach Donn recovered and challenged Kildare by throwing a bridge over the Shannon in 1506. He also developed his position politically by arranging the marriage of his son, Conchobhar O'Brien (qv), to Annabella, daughter of MacWilliam Burke. Furthermore, his daughter, Mór, was married to her father's important vassal Donnchadh O'Brien. Four years later (1510) Toirdhealbhach Donn had his revenge, mauling Kildare's army in Limerick. In 1516 Toirdhealbhach allied with Sir Piers Butler and the O'Carrolls to rout Sir James of Desmond and the MacCarthys at Lough Gur. Thereafter he faded into the background, allowing his sons greater roles in the government of Thomond. Most notably, the O'Briens realigned themselves with the Kildares. In 1522 they campaigned with other Munster Irish into Ulster for Conn Bacach ÓNeill (qv), the Kildare ally, against Manus O'Donnell (qv). After O'Neill's defeat, they made peace with O'Donnell and returned home. Also the O'Briens became involved in the feud of Gerald FitzGerald (qv), 9th earl of Kildare, with Sir Piers Butler throughout the 1520s. Added to this was the reality that the competing spheres of the O'Briens and the Butlers were grating against each other in Tipperary. In the course of this struggle Toirdhealbhach Donn's son Tadhg was shot dead by the Butlers in 1523. On the death of Toirdhealbhach (1528 ), his son Conchobhar became king of Thomond while another son, Donnchadh, became tánaiste. By his two wives, this king of Thomond had four sons and one daughter.