O'Neill (Ó Néill), Eóghan (d. 1456), king of Tír Eóghain, was one of the sons of Niall Óg O’Neill (qv), king of Tír Eóghain, and his wife and first cousin Una, daughter of Domhnall O’Neill. On 21 July 1399 Eóghan is first mentioned in the annals, killing Cathal Maguire. His action was set against the background of his father's partially successful campaign to subdue the Ulster nobility. Eóghan first married Caitríona, daughter of Ardghal MacMahon of Oirghialla. She was the mother of his eldest son, Énrí (qv). However, Eóghan's life was transformed by the events of 1403, when his father died and within a few months his elder brother Brian Óg, the new king, died from smallpox. These deaths allowed Eóghan's cousin Domhnall Bog O’Neill (qv), son of Énrí O’Neill (qv) (d. 1392), to seize the kingship (1404). In the years that followed, Eóghan cultivated his links with the Maguires of Fermanagh and the MacMahons, and consolidated his position in western Tír Eóghain. And in 1410 his MacMahon allies captured Domhall Bog, sparking an O’Neill civil war. Four years later Domhnall Bog's brothers had their revenge, capturing Eóghan and forcing him to authorise the release of Domhnall Bog in return for his own freedom.
In 1419/20 Eóghan, with the O'Donnells, Maguires, and Brian MacMahon (d. 1442), expelled Domhnall Bog and forced him to flee to the English of Down. Although a peace was agreed within a year and Domhnall Bog was restored to his kingship, Eóghan was now the real power in the Tír Eóghain dynasty. However, his path to power was still troubled. In 1421 he was captured by the O'Neills of Clandeboye on his way to meet James Butler (qv), 4th earl of Ormond. His captivity lasted a year till his wife and sons raised his ransom. Having gained his liberty, Eóghan quickly allied with Niall Garbh O’Donnell (qv), king of Tír Conaill, and together they expelled Domhnall Bog and forced the Clandeboye O'Neills to submit. Following their successes, they brought several other Ulster lords on a campaign to invade north Connacht. In 1423 their victories mounted through their defeat of the English of Louth, leading to their extraction of a black rent from Dundalk, and their rout of the justiciar's army. The same pattern followed in 1424 when they defied Ormond's campaign into Ulster. During Christmas of that year Eóghan and the Ulster nobility came to Trim castle to submit before Edmund Mortimer (qv), earl of March and Ulster. But Mortimer's sudden death (18 January 1425) encouraged Sir John Talbot (qv) to exploit the situation and arrest the Ulstermen. Their confinement was brief, as Ormond, the new lieutenant, released them in return for hostages and ransoms. In his indenture with Ormond (23 July 1425), Eóghan significantly acknowledged that he was a vassal of the duke of York (qv), Mortimer's heir.
On his return to Tír Eóghain, Eóghan gradually realigned himself with Domhnall Bog against the O'Donnells. Indeed, Niall Garbh's instigation of the Clandeboye O'Neills against Eóghan (1427) signalled the end of their alliance. This marked his rebuilding of his father's authority over the province, adding the allegiance of the O'Reillys (1429). In 1430 he again attacked Louth and forced Dundalk to give him a black rent before taking the submissions of the Irish lords of Offaly, Westmeath, and Longford. The rise of Eóghan, however, presented Niall Garbh with a serious challenge. In 1431 a conference between the two collapsed, leading to continual warfare. During 1432 Eóghan at last succeeded Domhnall Bog as king of Tír Eóghain and determined to break the O'Donnells. Two years later, he got his chance. While Niall Garbh was helping the MacQuillans of the Antrim Glens, Eóghan hired a MacDonnell fleet from the Western Isles to raid the Antrim coast. When Niall Garbh decided to leave the MacQuillans to resist the MacDonnells, the result was predictable: the MacQuillans were destroyed. After this success, Eóghan ordered the fleet to sail directly to Donegal while he took the overland route. Eóghan's directness left Niall Garbh and his army stranded in east Ulster, exposing the vulnerability of Tír Conaill's defenders. On his approach, Fionnuala, Niall Garbh's wife, and his brother Neachtan O’Donnell (qv) sued for peace without consulting their king.
Eóghan's nimbleness humiliated Niall Garbh, and even though they concluded a peace at Caoluisce the O'Donnells’ embarrassment was profound. In December 1434 Eóghan and Niall Garbh campaigned into Louth, where the latter was conveniently captured by the English of that county, and imprisoned till his death (1439). Niall Garbh's demise and the subsequent weakness of the O'Donnells allowed Eóghan and his son, Énrí, to expand their power throughout Ulster. In 1441 they submitted to Ormond, which probably explains the presence of English troops in their ranks during their rout of Neachtan O’Donnell (1442). This brief alliance did not prevent Eóghan from ruthlessly extracting black rents throughout the 1450s from the English of Louth. Also, his regional dominance is evident when Eóghan in 1446 appointed Aodh MacMahon (qv) as king of Oirghialla. Furthermore, Eóghan responded to the elevation of Fearghal O’Reilly to the kingship of Breifne by the colonists (1449) by making Seaán O’Reilly (qv) king of Breifne. In contrast, his campaigns to conquer the Clandeboye O'Neills were decidedly less successful.
By the end of the 1440s Énrí was firmly established as Eóghan's heir. This can be seen in 1449, when (with the consent of Eóghan) Énrí confirmed an indenture at Drogheda (27 August 1449) with the lord lieutenant, Richard (qv), duke of York. Énrí’s prominence again is evident in his role in the negotiations between Aodh MacMahon and Eóghan in 1452. That year Aodh quarrelled with Eóghan and joined the English to thwart the O'Neill attack on the Fews of Armagh, inflicting heavy losses on Eóghan's gallowglass. This enraged the old king but Énrí’s diplomacy prevented the outbreak of war, allowing Aodh time to submit. Eóghan's incursions into Armagh that year also earned himself and his second wife, Evelina Barret, the censure of the archbishop of Armagh. The only significant O'Neill reverse that year was Ormond's invasion of Tír Eóghain. This was the culmination of a series of events. During that year Énrí deserted his wife, Gormflaith MacMurrough, and had Neachtan O’Donnell murdered. Adding insult to injury, Énrí slighted the MacMurroughs, Butlers, and O’Donnell by marrying the murdered man's wife. This act enraged Gormflaith's uncle, Ormond, who compelled Énrí to put away his new wife and take back Gormflaith.
What Eóghan thought of his son's risky diplomacy is unknown, but he annoyed Énrí by clinging to the kingship of Tír Eóghain. By summer 1455 Énrí had had enough and deposed his father, paving the way for his inauguration in July. Eóghan's removal on grounds of infirmity was confirmed in August by Archbishop John Mey (qv) of Armagh. He did not have long to live, dying peacefully in 1456.