O'Connor Faly (Ó Conchobair Failge), Muirchertach (d. 1305), king of Offaly, was probably the eldest son of Muirchertach O'Connor Faly; the identity of his mother is unknown and no record of his childhood or early manhood survives. From the 1260s relations between his dynasty and the FitzGerald barons of Offaly became steadily more tense and fractious, until in the early 1270s they broke down, ultimately causing a complete collapse of seigniorial and royal authority in west Leinster and the midlands. From fragmentary evidence it appears that a number of factors contributed to the crisis: a meteorological disaster that destroyed the economic basis of Irish life, a feud between the MacGillapatricks (Meic Gilla Pátraic) of Ossory and the O'Dempseys (Uí Dimusaig) of Clanmalire in 1271, and a prolonged vacancy in the barony of Offaly. By the following year war between the O'Connor Falys and the colonists had engulfed Offaly. The intensity of the conflict forced the justiciar, Maurice fitz Maurice FitzGerald (qv) (d. 1286), to take an army there in 1272 to quell the disturbances, but his expedition failed to pacify the O'Connor Falys and the war raged on until 1279–80. It is difficult to establish what role Muirchertach played, but by 1280 he had become king of Offaly, forming an effective partnership with An Calbach O'Connor Faly (qv), his younger brother and principal commander.
The cessation of hostilities was only temporary, and the intention of the brothers to continue their struggle against the colonists was signalled by their alliance in 1282 with Cairbre mac Airt O'Melaghlin (qv), king of Meath. From early in his reign Muirchertach was more a political than a military figure, and left martial affairs to his brother. The renewal of the conflict is noted in July 1283, when the Offaly territory of Oregan rendered only half of its annual dues as a result of the war of the Irish. The capture of the FitzGerald manorial caput of Lea castle by the O'Connor Falys and the O'Melaghlins in June 1284 prompted a colonial counter-offensive, coinciding with the coming of age of Gerald fitz Maurice FitzGerald (qv) (d. 1287), heir to the barony of Offaly, in 1285. The Irish routed the colonial forces in a series of encounters, culminating in their defeat of fitz Maurice in summer that year. Despite these successes, the O'Connor Falys were forced to sue for peace in 1286, but the region slid back into war in summer 1288, and again the O'Melaghlins and the O'Connor Falys won decisive victories against several colonial armies between 1288 and August 1289. A strong offensive into Offaly, led by Archbishop John Sandford (qv) of Dublin, forced the capitulation of the O'Connor Falys, who remained at peace until 1292. Thereafter hostilities broke out again and a state of continual conflict with colonial magnates continued into the early years of the fourteenth century.
In this period of expansion into Kildare and Meath, Muirchertach seems to have taken a secondary role, leaving An Calbach as the effective leader of the dynasty. In a dangerous development for the colony, in 1297 the O'Connor Falys began to form an alliance with the MacMurroughs (Méic Murchada) of east Leinster, encircling the colonial heartlands along the Barrow corridor and Kildare. To alleviate the growing Irish pressure on the beleaguered settlers, Peter Bermingham (qv) of Carbury and perhaps John fitz Thomas FitzGerald (qv) of Offaly conceived a plot to murder the O'Connor Faly leadership. Muirchertach, An Calbach, and another brother, Máel Mórda O'Connor Faly (c.1265–1305), with a considerable contingent of followers, were invited to a banquet on 13 June 1305. In the middle of the feast, Bermingham's retainers attacked the unarmed Irishmen; every one was killed, and the crime became one of the most notorious of medieval Ireland. One source provides a motive for the murders, stating that the Irishmen were part of a plot to kill several prominent colonial magnates and that Bermingham's actions were preemptive. For his bloody deed Bermingham earned a princely reward of £100 for the heads of Muirchertach, Máel Mórda, and sixteen others, and became a hero in colonial literature. Only one of Muirchertach's sons seems to have survived him: Muirchertach Óg O'Connor Faly (qv).