Niall Glúndub mac Áedo (died 919) was a King of Aileach and later High King of Ireland who reigned in the late 9th and early 10th century. He was the son of Áed Findliath and the half-brother of Domnall mac Áeda, with whom he shared kingship of Aileach.
Around 881, Viking jarlskona Eivor Varinsdottir visited the settlement of Aileach, under the guise of a Viking from the settlement, Donegal. Under Flann's orders, Eivor had to see what the situation was with the Kings Niall and Domnall and her cousin, Bárid mac Ímair, King of Dublin. Upon her arrival, Eivor saw the Kings Niall and Domnall and her cousin, drunk, as the kings asked her what prompted her travels to Aileach. Meanwhile, Bárid and Eivor created coded messages within their conversation with the kings as she lied, saying she was invited. From the conversation with the kings, Eivor found out from King Niall that Bárid and Flann's people were being held as hostages at the village's ringfort. As Bárid kept the kings busy, Eivor released the prisoners and took over Aileach from the inside.
Bárid took the kings as hostages as Eivor entered back into the kings' room. Both Eivor and Bárid questioned the kings of their actions, where Domnall admitted to being told by the Abbot Eogan mac Cartaigh. Reluctant to speak, under fear of Eogan, Niall confirmed Domnall's words under fear of Flann's wrath. Both Kings Niall and Domnall admitted that Eogan was building an army to march to Clogher. Warranting a need of alert, Bárid sent a message to King Flann of the recent events while he dealt with Kings Niall and Domnall as his hostages.
Around 882, after the defeat and death of the Ollamh Druid, Abbot Eogan, King Niall was one of the attendees of a meeting in Durrow that summoned High King Flann. In the meeting, an initial decision was made to enforce a druid inquisition after the recent events with Eogan and the Children of Danu. This decision was highly opposed from Flann's poetess, Ciara ingen Medba, who stormed out, while King Niall and the other kings agreed.
Behind the scenes
Niall is a historical character in Assassin's Creed: Valhalla's Wrath of the Druids DLC. Historically, Niall only began to rule alongside Domnall in the early 9th century, becoming sole king of Ailech in 911. In the events of the DLC however, which takes place between 879 and 882, Domnall and Niall are portrayed as co-kings of Ailech.