The Battle of Moira, known archaically as the Battle of Magh Rath, is fought on June 24, 637, near the Woods of Killultagh, just outside the village of Moira in what becomes County Down. The battle pits the Gaelic High King of Ireland Domnall II against his foster son King Congal of Ulster, supported by his ally Domnall the Freckled (Domnall Brecc) of Dál Riata.
The battle is allegedly the largest battle ever fought on the island of Ireland, and results in the death of Congal and the retreat of Domnall Brecc. The battle is caused as a result of the invading Gaels spreading out from Galway Bay. The Gaels have fled France and Spain to escape the Roman invasion of those areas. The Gaels are later to be known as Irish but are not native to the island. The native people of Ulster have been pushed into an area the size of two counties in what is now Antrim and Down.
Congal first establishes his power base in Dál nAraidi, where he becomes King before being recognised as King of Ulster in 627. His ambitions soon come into conflict with Domnall II, who becomes High King of Ireland in 628. Ironically, Domnall II rises to such a position because Congal has defeated and killed the previous High King, Suibne Menn, in a previous battle.
Domnall continues to press the rivalry with Congal very quickly. In 629 the two kings engage each other at the Battle of Dún Ceithirn in what is now County Londonderry. On that occasion Congal is defeated and Domnall is left unchallenged as the High King.
Throughout the 630s, Domnall continues to wage war on his rivals in the Uí Néill clan. In 637, however, Congal once again rises to challenge the Ard Rí, and enlists the help of Dál Riata to do so. The two forces meet just east of Lough Neagh.
Little is known about the actual battle itself. The armies of both Domnall II and Congal are primarily made up of warriors native to Ireland. However, Domnall I of Dál Riata brings a more varied force to the fight. His army included Scots, Picts, Anglo-Saxons, and Britons (Welshmen). At least one side has a substantial cavalry force.
There is reason to believe that the battle might have lasted a week, at the end of which the defeated force flees towards the woods of Killultagh. The forces of Ulster and Dál Riata are defeated, with Domnall of Dál Riata forced to flee north to his kingdom’s holdings. Congall is killed in the course of the battle.
The scale of the battle is confirmed in the 19th century when the railway line in Moira is being constructed. Thousands of bodies of men and horses are excavated. When one considers that the survivors probably numbered quite considerably more, then the reputation of the scale of the battle becomes obvious.