Niall Glúndub mac Áedo (modern spelling: Niall Glúndubh mac Aodha) (died 14 September 919) was a 10th century Irish king of the Cenel nEogain and High King of Ireland. While many Irish kin groups were members of the Ui Neill, tracing their descent from Niall of the nine hostages (Niall Noígiallach), the O’Neill dynasty took their name from Niall Glúndub rather than the earlier Niall. His mother was Mael Muire, daughter of Kenneth MacAlpin, King of Scots.
Son of Aed Finliath, Niall is first recorded succeeding his brother Domnall mac Áedo as King of Alech upon his death in 911. Extending his control to neighboring kingdoms, Niall defeated the Kings of Dal nAraidi and Ulaid at the Battles of Glarryford (in present day County Antrim) and Ballymena before his defeat by high-king Flann Sinna mac Mail Sechnaill of the Clan Cholmain Uí Néill at the Battle of Crossakiel (in present day County Meath). Following Flann's death in 916, Niall succeeded him as High King of Ireland.
It was during his reign in which he would reestablish the Oenach Tailteann, a traditional gathering of Irish clans.
Opposing the invasion by the Ui Imair during the next several years, Niall's forces fought a large battle against a group of Northmen in the summer of 917, ending inconclusively.
With the support of the clans of Leth Cuinn (Northern half of Ireland), the Uí Néill eventually acknowledged his claim to the throne. Despite his continued offensive against the Northmen however, they would continue to settle the area in large numbers, establishing strongholds in Dublin and various ports on the eastern coast.
Continuing to wage war against the Northmen, Niall advanced towards into Leinster, supported by the Uí Néill clans, the Airgialla and the Ulaid.
However, his forces were decimated by the Northmen under Sihtric Caech as Niall was killed, along with twelve other chieftains, at the Battle of Kilmashoge (near Rathfarnham in County Dublin) on 14 September 919.
Another account in the Anglo Saxon Chronicle has Sihtric Caech slaying his brother Niall in 921. Niall Glúndub was succeeded as High King by Donnchad Donn mac Flainn, son of Flann Sinna, and as King of Ailech by his son Muirchertach mac Neill, "the Hector of the Western World".