Battle of Bellaghmoon
908 CE.
Shown on the map at 52.877°, -6.885°.
The Battle of Ballaghmoon (Irish: Cath Bealaigh Mughna) took place on 13 September 908 at Ballaghmoon, near Castledermot in the south of modern County Kildare. It pitted the forces of Cormac mac Cuilennáin, king of Munster against an alliance comprising the forces of Flann Sinna, High King of Ireland, Cerball mac Muirecáin, king of Leinster, Cathal mac Conchobair, king of Connacht, and Cellach mac Cerbaill, king of Osraige. It ended in a decisive victory for the alliance led by Flann Sinna. The Conflict arose after Cormac mac Cuilennáin assembled an army to campaign against his eastern neighbour, Leinster, whose king Cerball mac Muirecáin was Flann Sinna's son-in-law and staunch ally. After this hostile act Flann in an alliance with Cathal mac Conchobair, king of Connacht, led an army into Leinster in order to help his son-in-law. Despite the men of Munster being outnumbered and peace talks being held between both sides, Cormac refused to surrender, it ultimately proved to be his undoing, he and Cellach mac Cerbaill were killed during the course of the battle.
Text from the Wikipedia article Battle of Ballaghmoon, licensed CC BY-SA 4.0.