O'Connor Faly (Ó Conchobair Failge), Muircheartach Óg (c.1290–1384), king of Uí Failghe (Offaly), was the eldest surviving son of Muirchertach (qv) (d. 1305), king of Uí Failghe. Nothing of substance is known of his early life; he cannot have attended the fatal banquet on Trinity Sunday, 13 June 1305, at which the host, Piers Bermingham (qv), had Muircheartach's father, at least two of his uncles, and their followers murdered. After this devastating blow the kingship apparently passed to Murchad, the late Muirchertach's brother, and then to his brother Máel Sechlainn, who died in 1327×1329. It is possible too that Cathaoir (d. 1367), son of Murchad, who was tánaiste, may have been king in succession to Máel Sechlainn, that is before Muircheartach Óg became king.
The O'Connor Falys were no longer the power they had been during the last decades of the thirteenth century and the beginning of the fourteenth. In 1333 the colonists cut forest passes through their territory with little hindrance, and in 1337–8 they were hard pressed to withstand a threat from the MacGeoghegans of neighbouring Kineleagh. Intent on restoring their power, the O'Connor Falys formed an alliance with the ambitious Conall O'Moore of Laoghis (Laois) and his family's former enemies, the O'Dempseys. At Easter 1346 they launched an attack on the earldom of Kildare as part of a series of offensives on the colonists in west Leinster and the midlands; they captured the castle of Lea and several other fortresses. Peace was temporarily restored by a colonial assault into the region during September 1346, after which Muircheartach Óg seems to have tried to build up his power more slowly by constructing a series of alliances with the O'Mores and O'Molloys of the midlands and the MacMurroughs of east Leinster. His success is suggested by the prestigious marriage in the 1350s or 1360s of his son Murchadh with Gormflaith, daughter of Art MacMurrough (qv) (d. 1362), king of Leinster.
When Muircheartach died in 1384, his son Murchadh O'Connor Faly (d. 1421), one of three recorded brothers and one sister, succeeded him. While his father had avoided conflict with the colonists, he waged war on them, driving them out of eastern Uí Failghe and pushing into Meath. He was aided by his marital alliance with his brother-in-law, Art Mór Mac Murrough Kavanagh (qv), king of Leinster, with whom on 16 February 1395 at Balgory, Co. Carlow, he submitted to the earl of Nottingham and – a month later – to Richard II (qv). Though willing to observe the terms of the peace concluded with Richard, Murchadh was forced back to war by a colonial drive to reclaim lands that he had taken from them. In this conflict Murchadh was supported by his eldest son, An Calbhach (qv), who in a skirmish in 1398 captured the earl of Kildare (qv). Under their dual leadership, the O'Connor Falys returned to the offensive, attacking regularly into Meath and Kildare. From time to time the colonial government was able to curb their advance, and Murchadh was forced to come to peace in September 1401; but after 1405 there seems to have been a sudden collapse in colonial resistance within the region, and Murchadh and his son swept deep into colonial territory. They scored major victories over the colonists in 1408, 1411, and 1414, and established a lucrative business from levying black rents. The rising prominence of the O'Connor Falys is underscored by the marriages Murchadh made for his children. An Calbhach married Mairghréag O'Carroll (qv), daughter of Tadhg O'Carroll of Eile (Ely), and Murchadh's daughter Joan married Murchadh, son of Donnchadh O'Byrne of Wicklow.
Around 1416 Murchadh O'Connor Faly significantly changed his tactics: in place of his hostility to the colonists, he now formed an alliance with the Butlers of Ormond, and he allowed An Calbhach's claims to the dynastic overlordship to be passed over in favour of those of his own brother Diarmaid (d. 1425/6). By consenting to Diarmaid's advancement, Murchadh effectively forestalled any division and preserved the unity that had made his dynasty the most feared in west Leinster and the midlands. Despite his advancing years, Murchadh remained active till his death in 1421, caused by a chill caught while on campaign. Diarmaid succeeded him as king, though under an arrangement concluded between him and An Calbhach the real power in Uí Failghe lay with An Calbhach, as the colonial administration acknowledged in March 1425. An Calbhach's succession was assured when Diarmaid died in 1425 or early in 1426, leaving one recorded son, Seaán Ballach.