O'Brien (Ó Briain), Brian Sreamach (d. 1399), lord of Thomond, was eldest of seven sons of Mathghamhain Maonmhagh O'Brien, lord of Thomond, and Una O'Connor. Nothing is known of his early life; but after his father's deposition of Diarmaid O'Brien as lord of Thomond in 1360, Brian Sreamach (‘the bleary-eyed’) became his father's designated successor. This is confirmed by Brian's first marriage to Sláine, daughter of Lochlainn Laidir MacNamara, and the joint submission by him and his father to Lionel of Clarence (qv) in summer 1365. Indeed, the youngster did not have long to wait to rule Thomond, as his father died in 1369. Despite his less than flattering sobriquet, Brian proved no slouch when it came to expanding O'Brien interests. Thus his arrival on the political stage was destined to be dramatic. On 11 July 1370, displaying great military skill and agility, he annihilated the army of Gerald FitzGerald (qv), 3rd earl of Desmond, at Monasteranenagh near Croom, Co. Limerick, taking the earl and many of his vassals prisoner. Not content with this, Brian redoubled his efforts and burned Limerick town before installing Shíoda Óg MacNamara as warden. But as soon as the O'Brien forces retired over the Shannon, the town burghers murdered the now isolated MacNamara. These defeats of the Munster colonists brought the justiciar, Sir William of Windsor (qv), and the bishop of Meath southward, and through a mixture of force and cajolery, they brought Brian, MacNamara, and MacCarthy of Cairbre to peace in mid December 1370.
Nevertheless Brian continued to build on his victory and sought to create a nexus of alliances countrywide. However, this hyperactivity led the council to report him to Edward III in the following year. By spring 1374 Brian and his ally Richard Óg de Burgh (qv) of Clanricard were wreaking havoc on the counties of Limerick and Cork. In response the council ordered Richard Óg to fight Brian. In September they had succeeded, as the former allies were now butchering each other. To complicate matters further Sir William put forward Brian's uncle and tánaiste Toirdhealbhach Maol O'Brien for the kingship of Thomond, and through bribes secured the support of O'Dea and O'Connor of Corcomroe. By October the council had also succeeded in turning MacNamara. Although Brian fought valiantly, he was expelled from Thomond early in 1375. But the parliament held at Kilkenny (18 June 1375) reported that Brian had found allies among the O'Connors of Connacht and conspired to destroy the land of peace. In August Sir William and Toirdhealbhach Maol were again compelled to campaign against Brian. Despite colonist success, the situation in Thomond was precarious as the war raged into summer 1376. Yet Toirdhealbhach Maol maintained himself against his nephew and attended the justiciar's court at Cork during 1377. By March the following year, however, Brian had expelled Toirdhealbhach from Thomond. The exile and his followers were eventually compensated by the FitzGeralds with lands in the Comeragh mountains of Waterford.
Brian was to spend the next few years consolidating his grip on his kingdom. But by the early 1380s he was back on the warpath. After the death (26 December 1381) of Edmund Mortimer (qv), justiciar of Ireland and earl of March, Brian devastated Limerick, Kerry, and Cork. Because of these new outrages a royal service was summoned to make war on him on 27 January 1384, but Brian brushed it aside. The feebleness of the government only encouraged him to meddle further afield through the making of alliances with Irish dynasties in Connacht, Thomond, and Leinster during summer 1384. It was probably about this time that Brian formed an alliance with the FitzGeralds of Desmond, marrying Mary, daughter of James FitzGerald. This alliance led to a prolonged period of peace in north Munster, allowing Brian time to further strengthen his regional hold there. However, this situation was rudely disturbed by the arrival of Richard II (qv) in October 1394. Brian was determined to oppose him and sent envoys to dissuade Niall Mór O'Neill (qv) from submitting. But Brian changed his tune and wrote to the king offering his fealty on 4 February 1395. On 12 February Richard issued letters patent empowering the earl of Nottingham to take liege homage of Brian and his vassals. Such was the effect of Richard's words on Brian that he travelled to Dublin to see the king, submitting before him at the abbey of St Thomas on 1 March. Three days later Brian and his vassals assembled in a field near Quin in Clare to hear the reading of the letters patent and to take the oath of allegiance. His belief in Richard was made apparent when he and his son Diarmaid, with Toirdhealbhach O'Connor Don (qv), repeated their pledges in Waterford on 30 April 1395. Brian's transformation to a model of conformity was sealed by his acceptance of a knighthood from Richard at a ceremony on Lady day in Christ Church cathedral.
Thereafter little was heard of Brian till his death in 1399 . He was succeeded as lord of Thomond by his younger brother, Conchobhar (d. 1426), who continued the policies of Brian, accepting Earl Gerald FitzGerald's son James fitz Gerald FitzGerald (qv) (d. 1462) as a foster brother for his own son Brian Óg. In 1407 James FitzGerald confirmed his alliance with the O'Briens by giving Brian Óg the wardenship of Carrigogunnell castle, and enfeoffed him with a territory in Limerick afterwards known as Pobal Bhriain. Conchobhar also maintained the old friendship with the de Burghs, marrying his daughter Sadhbh to Walter de Burgh. However, he faced challenges to his kingship from the sons of Brian, led by the eldest, Tadhg (qv). By 1415 Tadhg was the effective power in Thomond, but he respected the titular position of his aged uncle. Conchobhar lingered till his death on Easter Saturday 1426, and was succeeded by Tadhg. This transitional figure was married to Maire, daughter of Toirdhealbhach O'Brien of Clonnagh; they had three sons and one daughter.