O'Donnell (Ó Domnaill), Domnall Mór (c.1180–1241), king of Donegal, was the son of Éccnechán O'Donnell (d. 1207), king of Donegal from the late 1190s. Nothing is known of his early years, but it seems he was at his father's side during Éccnechán's struggle for the kingship. In comparison to the Ó Maeldoraid, Ó Canannáin (O'Cannons), and the other dynasties of Donegal, the O'Donnells were a relatively new power within the kingdom. Their opportunity arose in the turmoil that followed the death in 1197 of the powerful Flaithbertach Ó Maeldoraid (O'Muldory) (1138–97) and the slaying of his successor, Echmarcach O'Doherty, by John de Courcy (qv) (d. 1219); in Echmarcach's place, a number of Tír Conaill nobles chose O'Hegney (Ó Éicnig) king of Fermanagh as king. Éccnechán and Domhnall Mor O'Donnell were to become close associates of Áed O'Neill (qv) of Tyrone (d. 1230), who challenged de Courcy's plans for the expansion of the English into Ulster. In 1199 O'Neill repulsed de Courcy, who had invaded Tyrone, and forestalled the expedition of O'Hegney to join forces with his supporters in Tír Conaill, leaving the way clear for Éccnechán to claim the vacant kingship of Donegal.
The O'Donnells were far from puppets of O'Neill: they controlled a large reservoir of military manpower and had the tactical acumen to deploy it. Their rising power was demonstrated in 1200 when they used a seagoing fleet of thirteen warships and a land army against the MacLoughlins (Méic Lochlainn) and their vassals. They were also extremely able diplomats, constructing a network of relationships with nobles of the north-west. Éccnechán continued the connections of the deceased O'Muldory with the O'Connors (Uí Chonchobair) of Connacht, notably the descendants of Ruaidhrí Ua Conchobair (qv) (d. 1198). The O'Donnells’ success caused their neighbours to fear them, and later in 1200 Éccnechán was attacked simultaneously by the MacLoughlins and Ualgarc O'Rourke (qv) (d. 1231), king of west Breifne. He routed the O'Rourkes at the battle of Leckmuldory near Belleek on the banks of Erne, then Éccnechán turned north to confront the invading MacLoughlins, whom he defeated, killing Gerrmaide O'Boylan of Dartry with his own hand. In 1206 and 1207 Éccnechán continued to expand his influence throughout the north-west: he plundered parts of Tyrone, and then, with a force that included his O'Boyle vassals and some exiles of the O'Connors of Connacht, raided the men of Fermanagh, carrying off a large prey of assorted livestock. Aengus Mac Gillafinnéin (MacLennan, MacAlinden) of Fermanagh (d. 1234) and his ally Niall MacMahon of Oriel pursued the raiders and caught them at Tuatha Rátha; some escaped, but most were killed, among them Éccnechán, who was cut down by Mac Gillafinnéin in single combat.
Most of the annals note that Domnall Mór now became king of Donegal, though the annals in Mac Carthaigh's Book state that Cathbarr O'Donnell took the kingship; the Annals of Ulster depict this Cathbarr and a Cormac O'Donnell as Domnall Mór's subjects, which is more probable. Like his father, Domnall Mór took care to establish his family firmly in the political landscape by creating and maintaining good relations with other princely families. He appears to have married twice. The identity of his first wife is unknown, but she seems to have been the mother of his sons Maelsechlainn O'Donnell (qv) (d. 1247) and Gofraid O'Donnell (qv) (d. 1258). His second wife was Lasairfína O'Connor (d. 1282), daughter of Cathal Mór Crobderg Ua Conchobair (qv) (d. 1224), king of Connacht, who was the mother of Domnall Óc O'Donnell (qv) (d. 1281). In spite of his diplomatic efforts, Domnall Mór's position was under constant threat from neighbouring leaders, particularly O'Neill, who in 1209 swept into the disputed peninsula of Inishowen, forcing Domnall Mór to recognise him as overlord. After a vicious but indecisive battle the two kings resolved their differences and made a pact to support each other against their English and Irish enemies, an agreement that Domnall Mór was called upon to honour the following year, when O'Neill wished to check the Dublin government's attempt to expand its influence by ringing Ulster with castles. At Cael Uisce on the Erne in 1210 O'Neill and Domnall Mór swooped on the castle builders, razing their work and killing their constable. O'Neill seems to have remained the senior partner in this alliance, for he was described as ‘Rí Conaill agus Eogain’ (‘king of (Tír) Conaill and (Tír) Eogain’) in 1211–12, but Domnall Mór proved his strength in the latter year by attacking the MacLoughlins with the help of the fleets of Thomas MacUchtry, earl of Atholl, and Raghnall mac Somhairle of the Isles, devastating Inishowen and sacking Derry.
A bold claim concerning Domnall Mór's rising power is found in the traditional account of his feud with the poet Muiredach O'Daly, retailed in the Annals of the Four Masters. According to this in 1213 Domnall Mór's steward, Fionn O'Brollaghan, was collecting tribute from Carbury in Sligo when he became embroiled in a row with O'Daly; the poet killed O'Brollaghan with an axe, then fled first to the protection of Richard de Burgh (qv) (d. 1243), then to Donnchad Cairprech O'Brien (qv) (d. 1242), and then on to Limerick. Domnall Mór pursued him with such tenacity that O'Daly was eventually forced to flee Ireland for Scotland, from where he attempted to pacify the king of Donegal's rage by composing three praise poems to him; his efforts met with success and Domnall Mór permitted the contrite O'Daly to return to Ireland and granted him lands on which to live out his days.
Domnall Mór inherited from his father a greed for plunder and spoils and the power that they brought with them. Although he was the son-in-law of Cathal Crobderg Ua Conchobair, he expanded into the traditional sphere of influence of the O'Connors. In 1219 he plundered much of Leitrim, Longford, and Cavan, taking the submissions of the O'Reillys and the O'Rourkes, before attacking Fermanagh. In 1223 he raided Connacht itself, marching to the hill of Croaghan before skirting the western midlands in search of hostages. By comparison with his O'Connor relatives, Domnall Mór had little to do with the Dublin government and took no part in its campaign against the Lacys and O'Neill in 1224. But after the death of Cathal Crobderg in June that year he upheld the rights of his O'Connor kinsmen to the Connacht kingship by supporting them against the descendants of Ruaidrí Ua Conchobair and O'Neill. This defiance earned O'Neill's enmity and in 1226 he campaigned into Donegal, forcing Domnall Mór to yield hostages, among whom was one of his sons.
In due course Domnall Mór's importance to the sons of Cathal Crobderg became clear. In 1227 Cathal Crobderg's son and successor King Aed O'Connor (d. 1228), king of Connacht, declared war on the government, burning Athlone, for which act he was expelled, with his brother Feidlimid O'Connor (qv) (d. 1265), by Richard de Burgh and the sons of Ruaidrí O'Connor to Donegal. As they returned from Domnall Mór's kingdom through the Curlew Mountains, the brothers were ambushed by their rivals and were once more forced to flee. Aed son of Cathal was murdered in Leinster the following year, while Feidlimid seems again to have taken refuge at the O'Donnell court, where he stayed for two years, plotting with Domnall Mór and Lasairfína how to depose the sons of Ruaidri. In 1230 Domnall Mór administered a blow to the kingship of Áed son of Ruaidrí Ua Conchobair (qv) by burning Mag n-Aí in Connacht, and later that year he and Feidlimid allied themselves with Burke to repel Aed's attack against Burke lands; Áed was driven into Tyrone and Feidlimid became king of Connacht.
Domnall Mór's ambition to be the greatest Irish king in the region seemed close to fulfilment when O'Neill died suddenly in 1230, but he was checked by the dramatic reemergence of the MacLoughlins under Domnall MacLoughlin (qv) (d. 1241). In 1231 Domnall Mór joined Aengus MacAlinden of Fermanagh, his father's killer, to attack Cathal O'Reilly (qv) of east Breifne (d. 1256): in a daring move, they placed a fleet on Lough Oughter, plundered O'Reilly's fort on Eanish island, and kidnapped his wife, the daughter of MacTiernan (Mac Tigernáin). In 1232, though, MacLoughlin made a counter-move, and with English help expanded from his base at Derry into southern Donegal, taking hostages of O'Boyle and O'Taircheirt. Domnall Mór retaliated by burning as far as Tullaghoge in Tyrone. MacLoughlin sailed with a fleet to ravage Mevagh near Kilmacrenan and Aughnish island on Lough Swilly, subduing Domnall Mór for a time and encouraging his neighbours to rise against him. In 1234 Aengus MacAlinden of Fermanagh raided Donegal, drawing an immediate response from Domnall Mór, who killed Aengus with his own hand.
In 1235 Feidlimid O'Connor, after another defeat at the hands of Richard de Burgh, again fled Connacht to the safety of Donegal. He submitted shortly afterwards to the justiciar Maurice fitz Gerald FitzGerald (qv) (d. 1257) and obtained the king's five cantreds in Roscommon, but in 1236 he was again banished to Donegal, where Domnall Mór gave him a sympathetic welcome. After Feidlimid returned to Connacht later that year, Domnall Mór campaigned across Ulster in a vain attempt to assert his overlordship, penetrating to Newry and taking the hostages of the Ulaid (the Irish of Eastern Ulster). But the conquest of Connacht seriously affected his position: to Hugh de Lacy (qv) (d. 1242) Burke granted the five cantreds of modern Sligo, including Carbury, and between 1237 and 1242 Lacy, in turn, granted Carbury, as well as his rights as earl of Ulster within Donegal, to the justiciar fitz Maurice. Thus Domnall Mór's kingdom of Donegal and neighbouring Tyrone became targets for FitzGerald and Lacy expansion, in pursuit of which they exacted hostages from the O'Donnells and MacLoughlins in 1238.
Far from pacifying or subduing the Irish, this interference only exacerbated the siege mentality of the Irish kings and made the situation more volatile in the longer term. In 1239 MacLoughlin defeated Brian O'Neill (qv), at that time Lacy's ally, at Carnteel, before expelling Lacy himself from Ulster after a ferocious encounter. The separate threats posed by MacLoughlin and the FitzGeralds along his borders constantly undermined Domnall Mór's power base. In his last years the king assumed the habit of a Cistercian and retired to the monastery of Assaroe, where he died in autumn 1241. He was succeeded by his son Maelsechlainn.