O'Reilly (Ó Ragallaig), Matthew (Matha) Ruad (d. 1282), son of Domnall O'Reilly (d. 1256), an O'Reilly noble, was a powerful king of East Breifne. Matthew Ruad first came to prominence in the aftermath of the calamitous O'Reilly defeat at Magh Slécht on 14 September 1256. Among the sixteen fallen O'Reilly nobles were Matthew Ruad's father and great-uncle, Domnall and Cú Chonnacht O'Reilly (qv), and his grandfather Cathal (qv). Briefly another Domnall O'Reilly, Matthew Ruad's great-uncle, tried to rally the dynasty. However, he was powerless to prevent the devastation of East Breifne by Áed O'Connor (qv) (d. 1274) and Conchobar O'Rourke (qv) (d. 1257) of West Breifne. Indeed, the O'Rourkes won a series of minor encounters against the O'Reillys in this period. On 29 November Domnall requested a parley with Conchobar O'Rourke, but the latter behaved treacherously towards him after this meeting, pursuing him throughout East Breifne. At this stage Domnall and Matthew Ruad appealed to Walter de Burgh (qv) (d. 1271), lord of Connacht, and the de Angulos to destroy West Breifne and Connacht. Such was the O'Reilly weakness that Áed O'Connor restored Conn 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, Matthew Ruad stepped forward and took the kingship. He quickly moved against his enemies, killing Conchobar O'Rourke, who had fallen out of favour with Áed O'Connor. Moreover, he ruthlessly blinded his own vassal Giolla Pátraic MacCreagh (Mág Fiachrach). However, Matthew Ruad had to acknowledge the O'Connor supremacy, rendering them hostages in 1258 and 1259. In the years that followed, the O'Reillys directed their attentions on the lands of the English of Meath, particularly those of the de Verdons. After the death of Áed O'Connor (1274), Domnall Óc O'Donnell (qv) (d. 1281) sought to assume his mantle of regional overlordship. Then he brought a great fleet on Lough Oughter and compelled Matthew Ruad to submit to him. During 1282 Matthew Ruad died peacefully and was succeeded by his brother, Fergal O'Reilly (c.1238–1293), of whom little is known except that he reigned till 1293, and that his daughter Áine married Tomaltach MacDermott of Mag Luirg (d. 1336). His end came when he was ambushed in 1293 by his rebellious MacTiernan vassals. In turn, Fergal was succeeded by another brother, Gilla Ísu Ruad O'Reilly (c.1242–1330), who first came to notice in 1283, joining with Brian MacMahon (qv), king of Oirghialla, to defeat and kill Áed Buide O'Neill (qv), king of Tír Eógain. The new king moved quickly to consolidate his position, taking the hostages of Brian Magauran (Mág Shamradáin) and MacGourkey (Mág Dorchaid) in 1295. He further developed his links with Magauran, forming a marriage alliance between their houses through the union between his son Matthew and Magauran’s daughter. The genealogies reveal that Gilla Ísu Ruad sired some thirteen sons. These he used judiciously, giving many of them English names and sending them to serve against the O'Byrnes in Leinster during 1295 and 1302. A major threat to Gilla Ísu Ruad's kingdom was the O'Connor branch known as the Clan Murtagh. During 1302 he led his troops to victory over them, killing their ally, the baron Peter Petit. But it was clear the pressure was mounting on the O'Reilly kingdom. In 1304 Gilla Ísu Ruad's son Matha (Matthew) was killed by the MacTiernans, and the following year the Clan Murtagh killed Pilib O'Reilly and 140 galloglass. Probably due to the dangers posed by the Clan Murtagh, Gilla Ísu Ruad again moved closer to the English. Between 1309 and 1310 Master William de la Ryvere acted as an envoy for the English to secure his neutrality in the ongoing war between the English of Meath and the Clann an Chaoich (Clankee) O'Reillys. Gilla Ísu Ruad's close links with the English were confirmed by the royal summons to campaign against the Scots in 1314 and 1315. However, it was clear that Gilla Ísu Ruad was weakening. In 1314 Ruaidrí O'Connor (qv) (d. 1316) and the Clan Murtagh invaded East Breifne and defeated Gilla Ísu Ruad at Drumlane. This defeat seems to have convinced him to retire, handing his lordship over to his son Máel Sechlainn O'Reilly (qv) (d. 1328). In retirement he took the cowl of a Franciscan and entered his own foundation of Cavan abbey, dying there in 1330.
AFM, iii; ALC, i; Ann. Conn.; AU, ii; Misc. Ir. Annals; Ann. Clon.; K. Simms, ‘Gaelic lordships in Ulster in the later middle ages’ (Ph.D. thesis, Dublin, 1976), ii, 370–407