Mág Uidhir (Maguire), Cú Chonnacht Óg (‘an Comharba’) (1480?–1537), lord of Fir Manach (Fermanagh), was son of Cú Chonnacht Mág Uidhir (head of the junior Enniskillen-based branch of the Maguires) and grand-nephew of Seaán Mág Uidhir, lord of Fir Manach 1486–1503. On the death of the latter the lordship reverted to a member of the family's senior Lisnaskea-based branch, Conchobhar Mór Mág Uidhir, but within a few years Cú Chonnacht Óg was recognised as the heir in waiting.
In this period the more powerful neighbouring lordships of Tír Eóghain and Tír Conaill were competing for dominance in Fir Manach, and Cú Chonnacht Óg tended to favour the latter. In 1518 he was defeated while raiding in Tír Eóghain and in 1522 he was obliged to submit to Con Bacach Ó Néill (qv), subsequently 1st earl of Tyrone, but in 1526 he led the Fir Manach contingent that accompanied Aodh Dubh Ó Domhnaill (qv), lord of Tír Conaill, on campaign in north Connacht. When Conchobhar Mág Uidhir died in 1527 Cú Chonnacht Óg was proclaimed chief of the Maguires by Aodh Dubh Ó Domhnaill, with whom he again campaigned in the same year. In 1531 he led a Fir Manach force into Tír Conaill at Ó Domhnaill's invitation in order to help curb the rising power of the latter's son, Manus O'Donnell (qv). In 1532 Cú Chonnacht Óg travelled to Drogheda with Aodh Dubh Ó Domhnaill, where both submitted to the lord deputy, William Skeffington (qv), and took part in Skeffington's expedition against Con Bacach Ó Néill.
Cú Chonnacht Óg Mág Uidhir was murdered on the island of Craghan in Lough Erne on 8 October 1537. He was first buried at Devenish, Co. Fermanagh, but was later reinterred in Donegal abbey. The lordship briefly reverted to the Maguires' senior line in the person of Giolla Pádraig Bán Mág Uidhir, but in 1540 he was deposed by Con Bacach Ó Néill, who appointed Seaán Mág Uidhir, son of Cú Chonnacht Óg, as lord. Cú Chonnacht Óg was married to Siobhán, probably a daughter of Seaán Ó Raghallaigh, chief of the O'Reillys. His other children included Siobhán, who married Matthew Ó Néill (qv), baron of Dungannon, and was the mother of Hugh O’Neill (qv), 2nd earl of Tyrone; and Cú Chonnacht Óg Mág Uidhir (Maguire) (1520?–89), who was installed as chief of the Maguires in 1566 with the backing of Seaán Ó Néill (qv), lord of Tyrone, who had deposed his brother and predecessor, Seaán Mág Uidhir, earlier in the year.
The new chief was obliged to recognise the overlordship of the O'Neills and to pay tribute to Seaán Ó Néill's successor, Turlough Luineach (qv), but he frequently complained about these exactions to the Dublin administration, with which he maintained good relations. Mág Uidhir surrendered Fermanagh to the crown in 1585 and had it regranted to him and his heirs in 1586, subject to an annual payment of £120 that was to begin as soon as his payments to the O'Neills ceased. The shiring of Fermanagh was envisaged in the grant and in 1587 a commission empowered ‘Sir Cochonaght Magwyre’, by then a knight, to execute martial law in the county. Mág Uidhir died on 17 June 1589, and an annalist recording his death praised his generosity to the church and poets. The former statement is verified by his foundation of a Franciscan abbey at Lisgoole in 1583–6 and the latter by the contents of ‘Duanaire Mhéig Uidhir’, a compilation of twenty-four bardic poems in his praise by professional poets who included Fearghal Óg Mac an Bhaird (qv) and Eochaidh Ó hEoghusa (qv).
Cú Chonnacht Óg appears to have married three times: first Nuala, daughter of Manus O'Donnell, lord of Tyrconnell; secondly, a woman named Bé Bhionn, about whom nothing is known; and finally Mairghréag, daughter of Seaán Ó Néill, lord of Tyrone. His children by his first wife included Aodh (qv), his immediate sucessor as lord of Fermanagh; those by his third wife included Brian, who received an estate of 2,500 acres at Tempo in the plantation of Ulster, and Cú Chonnacht Óg Mág Uidhir (Maguire) (1570?–1608), last of the Maguire chiefs.
When Aodh Mág Uidhir, lord of Fermanagh and Cú Chonnacht Óg's half-brother, was killed in 1600, the chiefship was contested by two claimants: Conchobhar Ruadh Mág Uidhir, a member of the Maguires' senior branch and the grandson of a former chief, and Cú Chonnacht Óg. Although Hugh O'Neill, earl of Tyrone, favoured Conchobhar Ruadh, ‘Red’ Hugh O'Donnell (qv) created a fait accompli by proclaiming Cú Chonnacht Óg as chief at a banquet attended by Tyrone and the rival claimants. In 1601 Cú Chonnacht Óg commanded a Fermanagh contingent that suffered heavy losses at the battle of Kinsale. On his return to Fermanagh he was opposed by Conchobhar Ruadh Mág Uidhir, who now enjoyed English support and had acquired the sobriquet ‘Mág Uidhir Gallda’. In 1603 Cú Chonnacht Óg was joined in Leitrim by a small force led by Domhnall O'Sullivan Beare (qv) that had marched from Munster, and together they defeated Conchobhar Ruadh. However, following the submission of Ruaidhrí O'Donnell (qv) and Hugh O'Neill to the crown, Cú Chonnacht Óg also submitted and accepted terms under which three of Fermanagh's eight baronies were assigned to Conchobhar Ruadh.
In 1607 Cú Chonnacht Óg left Ireland and, having secured financial assistance in the Spanish Netherlands, bought or chartered a ship and sailed from Dunkirk to Rathmullan in Lough Swilly. There he was joined by the earls of Tyrone and Tyrconnell and about one hundred of their followers before the ship set sail for France on 4 September 1607, an event that has become known as ‘the flight of the earls’. Those who departed included Tadhg Ó Cianáin (qv), hereditary chronicler to the Maguires, who kept a record of the party's journey to Rome.
In June 1608 Mág Uidhir left Rome and set out for Spain. His ship put in at Genoa, where he died of fever on 12 August 1608. He was married to a niece of the earl of Tyrone, and had one son, Donnchadh, whose fate is not known.