O'Neill (Ó Néill), Conn (d. 1493), king of Tyrone, the eldest son of Enrí O'Neill (qv) (d. 1489), prince of Ulster, is first mentioned in the annals in 1470, when he avenged the murder of Ruaidhri Bacach O'Neill by cutting down six of his killers in battle. In the following year he ably supported his father at the siege and eventual capture from Clann Airt O'Neill of Omagh castle. In the face of the growing power of the earls of Kildare, Conn sought to neutralise the threat by marrying, about 1480, Lady Eleanor FitzGerald, daughter of Thomas fitz Maurice FitzGerald (qv) (d. 1478), 7th earl of Kildare. In the midst of an O'Neill civil war in 1481 Conn was taken by Conn O'Neill (qv), lord of the Clandeboy O'Neills, and transferred into the hands of Aodh Ruadh O'Donnell (qv), lord of Donegal; he remained in captivity for two years and was released only after long negotiations. On his return to Tyrone in 1483 he succeeded his aged father as king and at once set about restoring O'Neill power in Ulster, by attacking the O'Donnells and the English settlers in the east; at the same time the internecine strife among the O'Neills continued and Conn also engaged the MacMahons (Meic Mhathghamhna) in Oriel. In 1488 he reached a peaceful settlement with O'Donnell and at the same time made terms with his dynastic rivals, the sons of Seán Buidhe O'Neill.
He then turned his attention to the O'Kanes (Uí Chatháin), taking hostages from them in 1489. This success may have encouraged him to revive the feud with O'Donnell: a stand-off in Donegal during winter 1490 was followed by renewed but inconclusive fighting early in the new year. The protagonists travelled to Dublin to place their dispute before the lord deputy, Gerald FitzGerald (qv) (d. 1513), 8th earl of Kildare, for arbitration, but not even his diplomatic skills could bring them to peace. It may have been this failure to reach a settlement that caused growing opposition to Conn's leadership from within O'Neill ranks. On 8 January 1493 Conn was killed by his younger brother Henry Óg O'Neill (c.1450–1498); his death led to a long and bitter succession race between Henry Óg and another brother, Domhnall Clárach O'Neill (c.1445–1509), who is first heard of when on 4 April 1492 he was captured by the MacMahons of Oriel and briefly imprisoned in Monaghan castle. Domhnall secured the support of O'Donnell for the kingship, while Henry Óg was backed by the O'Kanes and the O'Mellans (Uí Mhealláin). The first armed confrontation between the rivals took place at Glasdrummond, near Dungannon, on 28 June 1492. The struggle continued for five years, during which Domhnall, though worsted, maintained an effective resistance to Henry Óg's attempts to extend his overlordship. It ended in spring 1497 when Domhnall resigned his claims to the O'Neill overlordship in exchange for considerable compensation.
His internal troubles resolved, Henry Óg now took up again the O'Neill conflict with the O'Donnells. In October he invaded Donegal, and killed Conn O'Donnell, his victory forcing Aodh Ruadh O'Donnell out of retirement to resume the kingship. On 21 July 1498 Henry Óg was assassinated among the O'Neills of the Fews by sons of his murdered brother Conn. Domhnall Clárach quickly gathered his kinsmen and his wife's people, the MacMahons, and marched to Dungannon to claim the kingship. Almost at once O'Donnell invaded Tyrone, defeating Domhnall at Cross Cavanagh; the sons of Conn then invited Kildare into Tyrone, who forced Domhnall, now reduced to a state of unprecedented weakness, to make peace. In 1500 the O'Donnells devastated Tyrone and burned Domhnall's town of Dungannon and the crannóg of Lough Laoghaire. In 1504 Toirdhealbhach O'Hagan (Ó hÁgáin) and his supporters stormed Dungannon castle, but when Domhnall retook the fortress he hanged the O'Hagans from the castle walls. His victory proved no deterrent to the O'Donnells, who crossed Tyrone's borders in 1505 and again in 1507, razing Dungannon on both occasions; his own vassals and kinsmen too became a threat to the king. In 1507 he led an expedition against Brian O'Gormley (Ó Gairmleadhaigh), before waging war on the sons of Conn; but his efforts were increasingly unsuccessful. He again submitted before Aodh Dubh O'Donnell (qv), with Conchobar Maguire (Mág Uidhir), at Enniskillen, but his capitulation did not bring peace. In 1509, at the behest of the sons of Conn, Kildare again campaigned in Tyrone; the kingdom was ravaged and Domhnall's castles of Dungannon and Omagh captured. Domhnall died on 6 August 1509. He was succeeded by Art O'Neill (qv), lord of the O'Neills of the Fews.