O'Brien (Ó Briain), Mathghamhain Dall (d. 1448 ), king of Thomond, was probably second son of Brian Sreamach O'Brien (qv), king of Thomond, and Sláine, daughter of Lochlainn Láidir MacNamara. Nothing is recorded of his childhood or youth. After the death of Brian (1399) the kingship of Thomond passed to his younger brother Conchobhar O'Brien, who at first enjoyed amicable relations with the sons of Brian. But within a decade of Conchobhar's accession serious trouble had emerged among the O'Brien dynasty. By 1409 fighting had broken out between Conchobhar and his sons and those of Brian. This engulfed Thomond and spread to the traditional vassals of the O'Briens. However, a truce was offered by Conchobhar to his nephews in 1410. This was accepted by Mathghamhain's elder brother Tadhg O'Brien (qv) (d. 1444), and by 1415 the sons of Brian were the most powerful faction within the kingdom of Thomond. Throughout this period Mathghamhain seems to have loyally supported his brother's cause. After Tadhg's accession to the Thomond kingship following the death of Conchobhar (Easter Saturday, 30 March 1426), relations steadily soured between the brothers. Over the course of the next decade they became estranged by this struggle for power. Finally in 1438 Mathghamhain had enough strength to topple his brother and seize the kingship for himself. His coup proved a success, but he spared the life of Tadhg. Mathghamhain reigned for five years, but sudden blindness cost him his prize in 1444. He was succeeded by another brother, Toirdhealbhach Bog O'Brien (1405–59), who was faced by the reemergence of dynastic feuding with the sons of Mathghamhain. In 1446 war broke out in Thomond and his enemies captured him after a bloody skirmish. However, the MacWilliam Burkes came to his rescue and forcibly released him. Now with Burke muscle behind him, Toirdhealbhach Bog was able to implement a settlement. This held till his death (1459), when a struggle emerged between his son Tadhg O'Brien (qv) (d. 1466) and Mathghamhain's son Donnchadh (c.1420–?). However, Tadhg proved too strong for Donnchadh, deposing him in 1461.
Sources
AFM, iv, 785, 801, 851, 869, 913, 939, 949; J. O'Donovan (ed.), ‘MacFirbis’ Annals’, Miscellany of the Irish Archaelogical Society, i (1846), 209, 214, 258, 261; Curtis, Med. Ire. (1938), 306; K. W. Nicholls, Gaelic and gaelicised Ireland (1972), 96, 156–7; NHI, ii, 578, 581–2; ix, 152–3; John O'Hart, Irish pedigrees, i (1989), 159–60