Niall Glúndub mac Áedo (modern spelling: Niall Glúndubh mac Aodha) was a 10th-century Irish king of the Cenél nEógain and High King of Ireland. While many Irish kin groups were members of the Uí Néill and traced their descent from Niall of the Nine Hostages (Niall Noígiallach), the O'Neill dynasty took its name from Niall Glúndub rather than the earlier Niall. His mother was Máel 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 Ailech upon his death in 911. Extending his control to neighboring kingdoms, Niall defeated the Kings of Dál nAraidi and Ulaid at the Battles of Glarryford (in present-day County Antrim) and Ballymena before his defeat by high-king Flann Sinna mac Maíl Sechnaill of the Clann Cholmáin 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 Óenach Tailteann, a traditional gathering of Irish clans. He was married to Gormflaith ingen Flann Sinna.