O'Neill (Ó Néill), Domhnall Bog (d.1432), eldest son of Énrí Aimhréidh O'Neill (qv) and Affric, daughter of Aodh O'Neill, was a much disputed provincial king of Ulster in the early fifteenth century. It seems that Domhnall became leader of his father's followers on the latter's death (16 May 1392). His territory was predominantly located in the western Tír Eógain marches facing the expansionist kingdom of the O'Donnells. Shortly after Énrí Aimhréidh's death, Domhnall's lands were opportunistically invaded by Toirdhealbhach O'Donnell (qv) (d. 1423). Domhnall vehemently resisted the O'Donnells, but was taken during the fighting and carried off to Donegal. This prompted his cousin and overlord Niall Óg O'Neill (qv) and the O'Connors of Sligo to mount a counter-attack into Donegal, ravaging O'Doherty's lands in Fanad. It seems that Niall Óg's pressure prised Domhnall from the clutches of O'Donnell; but it was clear that splits were quickly emerging between Domhnall and his cousin. This was never more obvious than during Niall Óg's negotiations for his submission with Richard II (qv). Such was the opposition from among the O'Neills and their vassals to such a course that Niall Óg called an assembly in spring 1395. And in late 1395 Domhnall assaulted Niall Óg's residence, capturing his wife and son Brian Óg O'Neill (qv) (d. 1403) before taking them to the English. This forced Niall Óg to ransom Domhnall's brother Brian Mór from his O'Donnell brother-in-law in order to exchange him for his own son Brian Óg. The political landscape in Ulster was further complicated by the assault of the lord lieutenant, Roger Mortimer (qv), earl of March and Ulster, on the Ulster Irish. This chaos brought advantages to Domhnall, who allied with O'Donnell in 1397 to maul Niall Óg's campaign army in an ambush in Donegal. However, Domhnall deserted O'Donnell and returned to the O'Neill fold in 1398, campaigning into Donegal that year. His skill at playing both sides ran out in 1400, when he was captured by the English and imprisoned in Dublin. During Domhnall's imprisonment, Niall Óg made rapid gains and extended his overlordship over much of the northern province. Indeed, the fear of Niall Óg's dominance may have prompted the Dublin government to release Domhnall. The price of his release was high. In late 1402 he was set free in exchange for twelve hostages, including his own son. He quickly repaid his debt to the English, leading his forces into battle with Niall Óg. However, Niall Óg's peace of 1401 with O'Donnell prepared him well for Domhnall's onslaught, devastating his country in 1403. But before Niall Óg could bring Domhnall to his knees, he died during harvest 1403. He was shortly followed by his son and successor Brian, who caught smallpox and died within months of his father.
With the door ajar, Domhnall confidently stepped in to fill the void, declaring himself provincial king. The first years of his reign went quite well, but disaster struck in 1410 when Brian MacMahon of Oirghialla, brother-in-law of Eóghan O'Neill (qv) (d. 1456), captured him. Eager to prompt his kinsman's claims to the provincial kingship, MacMahon sold Domhnall to Eóghan, who then sent his captive to Maguire of Fir Manach for the duration of his imprisonment.
Domhnall's luck changed again in 1412, when his brother Aodh escaped from Dublin castle. Once back in Ulster, Aodh devoted all his energy to obtaining Domhnall's freedom. In 1414 Aodh and his allies finally captured Eóghan and forced him to order Domhnall's release in exchange for his own liberty. Domhnall's restoration and kingship were further confirmed when the lord lieutenant, John Talbot (qv), took his submission and that of the Clandeboye O'Neills in 1415. Throughout this period Toirdhealbhach O'Donnell was steadily eroding Domhnall's much disputed kingship, establishing a hegemony over the O'Connors of Sligo, O'Kanes, O'Rourkes of Breifne, and Maguires of Fir Manach. Moreover, Domhnall's position became further exposed when Brian Ballach O'Neill (qv) of Clandeboye broke with him. Indeed, the full extent of the ambit of O'Donnell power in Ulster was demonstrated in 1418 when Toirdhealbhach's son and heir, Niall Garbh O'Donnell (qv), with Eóghan, drove Domhnall from Tír Eógain over the Bann into the Antrim glens. There Brian Ballach determined to eject Domhnall before he established a foothold, ambushing his forces. In 1419 Toirdhealbhach, with Eóghan, again forced Domhnall to flee Tír Eógain, this time to the English. Against the odds Domhnall was to reestablish himself in western Tír Eógain. Clearly, the Dublin government was intent on maintaining him as a counterpoise to the rising power of Eóghan and his O'Donnell allies. In 1420 the lord lieutenant, James Butler (qv), 4th earl of Ormond, displayed this commitment by placing the hostages of Magennis of Iveagh in the hands of Domhnall. But Toirdhealbhach and Eóghan again moved quickly to stall Domhnall's resurgence in Ulster, forcing him into exile with Brian O'Connor (qv) of Sligo before confiscating two cantreds in western Tír Eógain and the crannóg of Lough Laeghaire. Domhnall struck back by requesting the O'Connors of Sligo to spoil Donegal while its army was campaigning in Tír Eógain. However, he had a change of heart and decided to join his enemies. In 1422 they invaded east Ulster to avenge Eóghan's capture by the Clandeboye O'Neills. Significantly, though, it was to Domhnall, not Eóghan, that the Clandeboye O'Neills gave their hostages.
With the death of Toirdhealbhach (1423), Niall Garbh became king of Donegal. The first act of his kingship was his plundering with Domhnall, Eóghan, and MacMahon of the English of Meath and Louth. The alliance with Domhnall was brief, as Niall Garbh helped Eóghan to expel him from Tír Eógain. In January 1425 Domhnall and many of the Ulster Irish came to see Edmund Mortimer (qv), earl of Ulster and March, at Trim castle. But Mortimer's death from plague prompted their arrest by Talbot. However, Domhnall, Brian Ballach O'Neill of Clandeboye, and MacQuillan were quickly able to negotiate their releases. After this nothing much was heard of Domhnall, indicating his rapidly declining political status and the rise of Eóghan and his sons. In January 1432 Domhnall and his son were killed by the O'Kanes while visiting their country.