14 September 919 AD: The King of Ireland at the time, Niall ‘Glundubh’ mac Aeda, (Niall ‘Black Knee’, son of Aed) raised an Army from amongst the the Gaels and marched to Dublin to retake it from the Vikings. On this day a great battle was fought outside the walls, the battle of Ath Cliath/Cell Moshamog, which was probably by the banks of the river Liffey at Islandbridge. The Irish were badly defeated and lost many of their warriors, King Niall himself being among the fallen. It was the Vikings greatest military victory over the Irish.
Niall was one of the Cenél nEóġain from what is now Co Tyrone in the North of Ireland. 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. But his reign was short lived as he met with defeat by the banks of the Liffey.
Though some place the battle at Kilmashoge, near Rathfarnham in County Dublin as a location, the Ford at Islandbridge is a more likely spot as Niall was advancing from the North and had to cross the Liffey to get around to the river's right bank to attack the city's walls. It is from him that the O'Neills of the North trace their ancestry.
Mournful today is virginal Ireland
Without a mighty king in command of hostages;
It is to view the heaven and not to see the sun
To behold Niall's plain without Niall
Annals of Ulster