O'Reilly (Ó Ragallaig), Cathal (c.1195–1256), king of Breifne, eldest son of Annad Ó Ragallaig, king of East Breifne, along with his brother Cú Chonnacht O'Reilly (c.1200–1256), developed their dynasty into a great power in the Cavan–Leitrim region. It is not known for sure when Cathal became the leader of his dynasty, but it seems he was the O'Reilly leader who was forced to surrender the crannog fortress at Lough Oughter to William Gorm de Lacy (qv) in 1220. From early on it was clear that Cathal was closely associated with the O'Connor kings of Connacht, fostering a son of Áed O'Connor, disputed king of Connacht 1224–8. Naturally, Cathal was to fight for Áed O'Connor against the sons of Ruaidrí O'Connor (qv) (d. 1198) and the de Lacys throughout the war of the Connacht succession of 1224–6. In 1226 he, with Cormac Mac Diarmata, king of Mag Luirg, killed Art O'Rourke before destroying the de Lacy castle of Kilmore. His attack on the O'Rourkes was in line with his wider plans to roll up the O'Rourke overlordship in Breifne, determining that his own dynasty replace them. He was to remain close to the sons of Áed O'Connor, giving them shelter and protection within his lands. On the other hand, he seems to have enjoyed a rather strained relationship with their uncle Fedlimid O'Connor (qv) (d. 1265), while the sons of Ruaidrí O'Connor hated him. However, Cathal's activities did catch the attention of Domnall Mór O'Donnell (qv) (d. 1241), king of Tír Conaill. Indeed, Cathal's endeavours earned him a massive O'Donnell naval assault upon his island home on Lough Oughter. In this raid the O'Donnells carried off his treasure, a favourite horse, and his then wife, Cacht, daughter of Mag Fiachrach. He was to marry again, reflecting his close links with the sons of Áed O'Connor. His new wife was Ragnailt (an O'Farrell), second wife of the deceased Connacht king.
Undeterred by O'Donnell's rebuke, Cathal continued to expand. In 1233 William Gorm de Lacy led a mixed force of the Irish and the English of Meath on a plundering raid into the O'Reilly kingdom. However, the O'Reillys, led by Cathal and Cú Chonnacht, overtook the plunder-laden raiders, slaughtering them as they retreated into Meath. Some time after this battle, Cathal surprisingly and amicably resigned his kingship in favour of Cú Chonnacht, whose new prominence gradually became clear; he blinded Diarmait O'Rourke in 1236. Indeed, Cú Chonnacht played an important role in the restoration of Fedlimid O'Connor to the Connacht kingship during 1236/7, defeating the sons of Ruaidrí O'Connor and the MacDermots of Mag Luirg. In comparison, Cathal became a patron of the new monastery on Trinity Island in Lough Oughter. Cú Chonnacht's ambition was to match that earlier exhibited by his brother, expanding O'Reilly influence throughout the region, and interfering among the MacDermots and the O'Rourkes. His feud with the MacDermots culminated when they killed his son Fergal (1240). This drew a savage reprisal from Cú Chonnacht in the form of a devastating raid on Mag Luirg (1241). Then he deposed Cormac MacDermot and installed Donnchad MacDermot as king. Cú Chonnacht still remained on reasonably good terms with Fedlimid O'Connor, and it was at his insistence that Cú Chonnacht in 1242 arrested Tadc O'Connor, Fedlimid's foster-son and nephew. When the deed was done, Fedlimid opted to leave Tadc in the custody of the O'Reillys. At an uncertain date in 1243–4, Cú Chonnacht released Tadc, who, once free, attacked Cormac MacDermot and kidnapped his own mother, Étaín (a MacCarthy). Tadc's plan seems to have been to marry his mother to Cú Chonnacht. On his return to Breifne, Cú Chonnacht suddenly arrested Tadc, blinding and castrating him. The annals state that this was done at the instigation of the Galls, most likely the de Angulos. The mutilation of Tadc broke the alliance between his family and the O'Reillys, setting the scene for later bitter hostility. In response Tadc's brother Cathal, who was fostered with the O'Reillys, attacked them. More serious for the O'Reillys was Fedlimid's personal intervention in the crisis, invading Breifne during 1244.
Thereafter Fedlimid seems to have reverted to his old policy of thorny cooperation with the O'Reillys. During the great disturbances in Connacht in 1247–50, Cú Chonnacht and Cathal seem to have remained fairly loyal to the Connacht king. In 1249 Fedlimid took refuge in Breifne to escape the wrath of Maurice FitzGerald (qv) (d. 1257) before escaping to Brian O'Neill (qv) (d. 1260), king of Tír Eógain. In 1250 Fedlimid returned from Tír Eógain to Connacht, passing through Breifne. Hereafter his relations with Cú Chonnacht dramatically declined. Much of this seems to have been inspired by the Connacht king's favouring of Conchobar O'Rourke (qv) (d. 1257) and his alliance with Brian O'Neill. In response, Cú Chonnacht and Cathal, with Eochaid Mac Mathgamna of Oriel, allied themselves more closely to Maurice FitzGerald, joining him that year on campaign in Tír Eógain and Donegal. In the course of these expeditions, O'Neill was forced to give up hostages to FitzGerald. While in Donegal they deposed and killed its king, Niall Ó Canannáin, O'Neill's protégé. Conflict with Fedlimid was still some way off, but it was steadily brewing. In 1252 it came when Murchad O'Fallon (Ó Fallamain), high constable of the O'Connor kingdom, was cut down by the O'Reillys at Fenagh in Leitrim. In the following year Cú Chonnacht, with Cathal O'Connor and Giolla-na-Naemh Ó Fergail, attacked Cathal MacReynolds (Mac Ragnaill), devastating his small Leitrim kingdom. This audacious attack was a direct affront to Fedlimid and his son Áed O'Connor (qv) (d. 1274). With this in mind Ó Fergail quickly went home, leaving his comrades. At Mohill in Leitrim, Áed O'Connor finally caught up with the O'Reillys and their allies, routing them. Still Cú Chonnacht and Cathal remained obdurate, threatening the plans of the O'Connors, who in 1255 entered Breifne and made Conchobar O'Rourke king of that land; it seems also that they deposed Cú Chonnacht and replaced him with Conchobar O'Reilly (c.1225–1257), Cathal's son. What effect this had on the O'Reillys is unknown, Conchobar appears to have stepped aside to allow Cú Chonnacht to return. That year Áed mac Fedlimid travelled north to O'Neill to reaffirm their alliance. Mindful of his difficulties with the O'Reillys, he apparently induced O'Neill to assault the O'Reillys as part of the bargain. This he did, but the O'Reillys stoutly resisted the O'Neill offensive. It was some time after this that Cathal's O'Farrell wife, Ragnailt, died (1256).
Surrounded by enemies, the O'Reillys turned to the English. In 1256 they were summoned by Walter de Burgh (qv) (d. 1271), lord of Connacht, to join his attack on the O'Connors and Conchobar O'Rourke. On their way to join the de Burghs, the O'Reillys were overwhelmed by the O'Connors and the O'Rourkes at the battle of Magh Slécht at Slieve Anierin on the borders of East and West Breifne. This defeat was devastating in its decisiveness, ending O'Reilly ambitions. Sixteen prominent O'Reilly nobles, including Cú Chonnacht, Cathal, and his son Domnall O'Reilly (b. c.1220), were among the fallen. Briefly another Domnall O'Reilly (d. 1283), brother of Cú Connacht and Cathal, tried to rally the defeated O'Reillys, but he could not stop the devastation of East Breifne by the victors. In particular, Domnall nursed a considerable grievance against Conchobar O'Rourke of West Breifne, the killer of his son Annad O'Reilly at Magh Slécht. On 29 November Domnall requested a parley with O'Rourke but the latter struck first, pursuing him throughout East Breifne. At this stage Domnall and his grandnephew Matthew (Matha) Ruad O'Reilly (qv) (d. 1282) appealed to Walter de Burgh, lord of Connacht, and the de Angulos to destroy West Breifne and Connacht. But the O'Reillys were so weak that Áed O'Connor restored Conchobar O'Reilly to the kingship of East Breifne in 1257. The new king was short-lived, dying before the end of the year. At this stage, Domnall seems to have allowed Matthew Ruad to assume the kingship. After that nothing is mentioned of Domnall till his death in 1283.