O'Brien (Ó Briain), Conchobar (d. 1268), king of Thomond, was son of Donnchad Cairprech O'Brien (qv), king of Thomond, and Sadb, daughter of Donnchad O'Kennedy (Ó Cennéidig). This somewhat hesitant king ruled over Thomond (1242–68) during a period characterised by endemic violence, caused by sustained colonist advances into Irish-ruled territories. Unfortunately for him, Thomond stood directly in the path of this expansion. Like his father, Conchobar attempted to keep his lands as a loyal tenant of the king, paying an annual rent and generally keeping the king's peace. But more grants to colonists along the west bank of the Shannon, and their establishment of settlements at Bunratty and Clare, conspired to place him in an impossible position. By 1257 he had had enough and authorised his able son Tadc to devastate the settlers. The colonists retaliated and launched a full-scale invasion of Thomond. The O'Briens fought back, but Tadc had lost his faith and patience with his temporising father. He was clearly a hard-line leader who was determined to show no mercy to the colonists. In 1258 he met his cousin Áed (qv) (d.1274) son of Fedlimid O'Connor (qv) and together they travelled to Cáeluisce and conferred with Brian O'Neill (qv). At this meeting they formed an alliance against the colonists and gave the high-kingship to O'Neill. What effect this had on Conchobar is impossible to tell, but Tadc was clearly trying to force his dynasty into opposing the colonists. The younger O'Brien did not live long enough to reap what he sowed, as he died in 1259. However, his example lived on and reinvigorated his father, who now carried the war to the settlers, destroying FitzGerald manors in Galway in 1260. He was to remain hostile till his death at the hands of his kinsmen in 1268. He was married to Mór, the daughter of his leading MacNamara (Mac Conmara) vassal.
Ann. Inisf, 353, 357, 359, 361, 363, 367; MacCarthaigh's Book, 101, 103; AU, ii, 319, 323, 327, 339; Ann. Clon., 242; AFM, iii, 327; Ann. Conn., 75, 125, 129, 131, 135, 151; Orpen, Normans, iv; NHI, ii, 120, 159, 252–3; G. Cunningham, The Anglo-Norman advance in the south-west midlands of Ireland (1987), 71; John O'Hart, Irish pedigrees, i (1989), 158–60; A. Nic Giollamhaith, ‘Kings and vassals in later medieval Ireland: the Ui Bhriain and the Mic Conmara in the fourteenth century’, J. F. Lydon (ed.), Colony and frontier in medieval Ireland (1995), 201–16