O'Neill (Ó Néill), Áed (d. 1230), king of Tír Eógain, was son of Áed O'Neill, disputed king (1166–77) of Tír Eógain, and was a powerful and influential ruler from the late twelfth century to his death. Nothing is known of his mother. He first came to the notice of the annalists by defeating five expeditions by John de Courcy (qv) into his country and by deposing the king of Tír Conaill (1199). In 1200 he repeated his defeats of the Ulster colonists, but his prolonged interest in the affairs of Connacht also began in this year, when Cathal Mór Crobderg Ua Conchobair (qv) (O'Connor) of Connacht fled to Tír Eógain. Once he had regained his strength, Ua Conchobair returned to Connacht, only to be exiled yet again. Once in Ulster, he toured the houses of O'Hegney (Ó hEighnigh) of Fir Manach and O'Neill in search of help before making his way to de Courcy. In 1201 he tried again with the support of O'Neill and O'Hegney. Again Cathal Carrach Ua Conchobair (qv) (O'Connor) drove them out, defeating them at the bridge of Es Dara (Ballysodare) linking land under the control of Tír Conaill with the rest of Connacht. O'Neill was forced to take refuge in a church and was not allowed to depart till he deposited hostages with Cathal Carrach. His misfortunes did not end there, as he was deposed from his kingship by Conchobar MacLoughlin (Mac Lochlainn). O'Neill's decline was brief: he was quickly restored after the death of MacLoughlin that year. But it seems that O'Neill's rise was greatly facilitated by the collapse of de Courcy's lordship of Ulster in the opening years of the thirteenth century and the subsequent loosening of the bands of colonial power. Indeed, O'Neill tried to expand his influence into Tír Conaill at the expense of the rising O'Donnell dynasty (1209), but after an indecisive but fierce battle the combatants resolved their differences and made an alliance.
On 20 June 1210 John I (qv) landed in Ireland, determined to decisively break the power of the de Lacys as well as Philip de Braose (qv). By the end of July the fighting was all but over. Its result was a resounding success for John, who now held the lordships of Meath, Ulster, and Limerick. John now laid plans to extend royal jurisdiction thoroughly into Meath and Ulster; thus the justiciar, John de Grey (qv), began a series of offensives against the Irish kings bordering Connacht. The primary targets for colonist expansion were the Irish-ruled territories in Ulster. Even before these intrusions into Ulster, the Irish there were already restless. Earlier, during John's royal progress, O'Neill feigned fealty before making off with much of the royal baggage train. Now, when the royal emissaries demanded that he renew his pledge of loyalty, his reply captured the hardening of Irish attitudes to the colonists: ‘Depart, O foreigners . . . I will give no hostages at all this time.’ Moreover, the humbling of Connacht by the army of Geoffrey de Marisco (qv) and the building of castles at Athlone, Clones, and Cáeluisce set the scene for an explosion. The imposition of these castles astride strategic arteries directly threatened the relative autonomy of the Ulster kings. By 1211 the pressure of this vice began to show, and Cáeluisce was destroyed by an Irish confederation led by O'Neill. However, in 1212 Archbishop Henry of London (qv), with the help of an O'Connor army, penetrated to Cáeluisce, where he began rebuilding the ruin. To the east the building of Clones was continuing apace despite determined attacks from Niall MacMahon (Mac Mathgamna) and O'Neill. Then the tide turned in Ulster in favour of the Irish. In an offensive coordinated by O'Neill, the castles at Cáeluisce (near Belleek) and Clones were destroyed, and the settlement at Carlingford was burnt (1212–13). After 1213 O'Neill remained for the most part at peace till 1221, when he joined the de Lacy enemies of Cathal Crobderg to attack Meath and burn north-west Leinster before fighting alongside the de Lacys against a colonial hosting in 1224.
In the aftermath of Cathal Crobderg's death (1224), the position of his son and successor Áed O'Connor was precarious. Not only did he face a gathering tornado of familial rivals and the de Lacys, but now Richard de Burgh (qv) pressed his claims to Connacht. By early 1225 the sons of Ruaidrí Ua Conchobair (qv) judged that the time was right to strike. Then the Irish nobility of Connacht voiced their dissatisfaction with Áed's government by inviting the sons of Ruaidrí and O'Neill into the province. Their invasion of Connacht quickly became a triumphal procession as Áed's support crumbled. Their success culminated in Toirrdelbach O'Connor's inauguration at Carnfree by O'Neill. However, the swift reaction of Áed's colonial allies compelled O'Neill to withdraw quickly from Connacht. This setback did not prevent his continuing interference in his neighbours's affairs, meddling in Tír Conaill (1226) before harbouring the sons of Ruaidrí again (1230). In 1230 the Four Masters described this great king on his death as the defender of Leth Chuinn against the English of Ireland, while the annals of Clonmacnoise laud him as the only banisher and extirper of the English. He was married to Ben Mide, daughter of O'Hegney (AU), and had three recorded sons.