O'Neill (Ó Néill), Niall Óg (d. 1403), king of Tyrone, eldest of the six sons and two daughters of Niall Mór O'Neill (qv) (d. 1397), king of Tyrone, and Beanmidhe MacMahon, played a role, even as a youth, in the politics of the kingdom. About 1370 Niall Mór divided Tyrone with his brother Domhnall and married Niall Óg to Úna O'Neill, Domhnall's daughter. Apart from this, however, nothing is known about Niall Óg until 1381, when, following the death of Edmund Mortimer (qv), earl of Ulster and lord lieutenant of Ireland, he began to emerge as a major figure among the O'Neills. Niall Mór promoted his son's rise, sharing the leadership with him and keeping him at his side. On 10 April 1384 they burned Carrickfergus to establish their dominance over most of the English of Ulster, and in summer 1385 they and the MacMahons (Meic Mhathghamhna) threatened Louth. They grew so confident that in 1387 Niall Óg built a house at Eamhain Macha, the legendary site of the palace of Conchobar (qv) son of Ness (Conor Macnessa), king of Ulster, for the entertainment of the poets and the learned classes from all over Ireland.
The advance of the O'Neills caused unrest, however, and in late 1389 Niall Óg was captured, while raiding cattle in the Cooley peninsula, by Edmund of London, constable of Carlingford castle and keeper of the Cooley lordship. Sir John Stanley (qv), justiciar of Ireland, sought to use this unexpected coup to check the O'Neills’ expansion, and at the end of January 1390 came north to negotiate with Niall Mór. On 20 February Stanley agreed to release Niall Óg; in return Niall Mór promised not to interfere with the bonaght of Ulster (the traditional rights of the earls of Ulster to billet mercenary troops upon the Ulster Irish), and gave up seven children as hostages, including the sons of his brother Cú Uladh O'Neill and Niall Óg's eldest son, Brian Óg O'Neill (d. 1404). English confidence in the agreement was confirmed by its being read at the parliamentary session of 21 March at Castledermot, Co. Kildare, and was put into effect when the hostages arrived at the castle of Roger Mortimer (qv), earl of Ulster and March, at Trim during Lent 1390 to begin their long imprisonment. In early March 1393 Niall Óg sent his wife Úna to Drogheda to negotiate the release of Brian. She was admitted to the town by James Butler (qv), 3rd earl of Ormond, and struck a deal, whereby Brian would be released in exchange for 1,000 cows and ten hostages.
The safety of the younger children caused Niall Óg and his father considerable concern and curtailed their expansionist activities, though it did not prevent Niall Óg from burning the suburbs of Dundalk in 1392, devastating the whole Carlingford peninsula, and killing Geoffrey White. Soon afterwards Niall Mór's health declined rapidly and he allowed Niall Óg to assume the leadership of the dynasty. The arrival of Richard II (qv) in Ireland in October 1394 and the success of his campaign in Leinster against Art MacMurrough (qv) (d. 1416/17) posed difficulties for Niall Óg. The O'Neills were anxious to avoid a confrontation, but at the same time were concerned by Richard's close association with Roger Mortimer. While Niall Óg remained aloof, he authorised his father to open negotiations with Richard. In a letter to the king, the old man explained the causes of his son's reluctance to submit as Mortimer's hostility and the renewed aggression of the English of Ulster; he acknowledged that he himself was a vassal of Richard and asked for his protection. On 20 January 1395 Niall Mór submitted before Richard in the Dominican house at Drogheda and presented letters patent from his son, complaining about Mortimer; he also undertook to surrender the bonaght of Ulster and all other rights due to the earl of Ulster. Niall Óg did not consider himself bound by his father's embassy, and in a letter to the archbishop of Armagh, dated 26 February, he wrote that he had come under strong pressure from Brian Sreamach O'Brien (qv) (d. 1399) and Toirdhealbhach Ruadh O'Connor (qv) (d. 1426) not to submit; there was also a considerable party among the O'Neills who opposed Niall Óg's submission. Despite this strong opposition, Niall Óg did homage before Richard at Dundalk on 16 March; in a letter to the king eight days later he wrote that his followers had reacted coolly to his submission and that he could no longer trust his own people, though his anxieties did not prevent him from travelling to Dublin to be knighted by Richard at Christ Church later that month.
The rapprochement between the king and the O'Neills did not deter Mortimer from asserting his rights in Ulster; Richard had decided to arbitrate over their competing claims on 24 June but left Ireland before this appointed date. He seems increasingly to have realised that the O'Neills were using him as a shield against Mortimer: in May, for example, Niall Óg wrote to him stating that Mortimer was intent on attacking O'Neill lands after Richard's departure. The king returned to England later that month. The O'Neills who opposed Niall Óg's agreement with the king were his nephews, Domhnall Bog O'Neill (qv) and his brothers, sons of Énrí Aimhréidh O'Neill (qv) (d. 1392). Late in 1395, they captured Niall Óg's wife, Úna, and son Brian and took them to the English. Niall Óg had to ransom Domhnall Bog's brother Brian Mór from Toirdhealbhach O'Donnell (qv) (d. 1423) and then exchange him for the release of his wife and son by Domhnall Bog.
In March and April 1396 Mortimer made his move. With the earls of Ormond and Kildare (qv) he attacked the O'Neills, burning Armagh and reestablishing control over much of eastern Ulster. In 1397 war broke out between Niall Óg and O'Donnell and his ally Domhnall Bog, beginning a long struggle for supremacy in Ulster. O'Donnell had been challenging Niall Óg for regional dominance since 1395, when he attacked Tyrone and Sligo. In general Niall Óg had the better of the struggle, though his army was mauled in Donegal in 1397. O'Donnell then plundered Fermanagh and Sligo unopposed, forcing Niall Óg to ally himself with the O'Connors (Uí Chonchobhair) of Carbury in Sligo. During a lull in the fighting, Count Ramón de Perelhos, the traveller and pilgrim, met Niall Óg during the Christmas festivities of 1397–8. In the account of his pilgrimage to St Patrick's Purgatory in Co. Donegal, de Perelhos gives a remarkable insight into the court of Niall Óg. He was greeted by the king's constable along with 100 horsemen on the edge of the O'Neill kingdom and was taken to the court, where he spent the Christmas holidays feasting and celebrating with Niall Óg and 3,000 followers.
Niall Óg resumed hostilities against O'Donnell early in 1398, defeating him in Donegal with the help of his new O'Connor allies from Sligo. His fortunes improved when Mortimer was killed on 20 July by the O'Byrnes (Uí Bhroin) and O'Tooles (Uí Thuathail) in Co. Carlow: the English and Irish of Ulster quickly made terms with Niall Óg, recognising his overkingship. He seems also to have made peace with Domhnall Bog and his brothers, as they campaigned together late in 1398, ravaging much of southern Donegal. These years were the high point of Niall Óg's career: he won further great victories over O'Donnell in 1399 and 1400, and over the English of Ulster in 1401. In that year O'Donnell concluded a treaty with Niall Óg at Caol Uisce in Fermanagh, which allowed Niall Óg to turn his attention to Domhnall Bog, whose territory he devastated in 1402. But before he could complete the defeat of his rival, Niall Óg died, at harvest time 1403. He was succeeded by his son Brian, but the new king caught the smallpox and died in early 1404 within months of his father.