The Battle of Moira, known archaically as the Battle of Mag Rath, was fought in the summer of 637 by 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 Dalriada.
The battle was fought near the Woods of Killultagh, just outside the village of Moira in what would become County Down. It was allegedly the largest battle ever fought on the island of Ireland, and resulted in the death of Congal and the retreat of Domnall Brecc.
Ireland in the period was a patchwork of petty statelets, fused together and driven apart by tribal loyalties, often given to a state of war. Other realms from across the water in Great Britain and in particular Scotland frequently became involved in the affairs of Ireland, notably Dalriada, which had come over from Scotland to occupy a substantial swathe of territory to the north of Lough Neagh. Indeed the tribal loyalties often spilled across the Irish Sea, where the same clans could be found on either side, especially in Scotland. Rivalries and alliances between the petty kingdoms changed frequently. For example Dalriada, which fought with Congal in this battle, had seen one of their kings killed by his brother at the Battle of Fid Eoin (either 629 or 630).
Congal himself had first established his power base in Dalaradia, where he became King, before being recognised as King of Ulster in 627. His ambitions soon came into conflict with Domnall II, who became High King of Ireland in 628. Ironically Domnall II only rose to such a position because Congal had defeated and killed the previous High King, Suibne Menn, (who was Domnall's distant cousin in the Uí Néill clan) in a previous battle.
Domnall at first launched a raid into Leinster in 628 to secure his authority as High King. Some primary sources state that Congal had initially become the Ard Rí following his defeat of Suibne Menn. It may be therefore that Domnall seized the position indirectly from his Ultonian rival. Regardless, the two had become enemies.
Domnall pressed this rivalry very quickly, and in 629 the two kings engaged each other at the Battle of Dún Ceithirn in what is now County Derry. On that occasion Congal was defeated, and Domnall was left unchallenged as the High King.
Throughout the 30's Domnall continued to wage war on his rivals in the Uí Néill clan. In 637, however, Congal once again rose to challenge the Ard Rí, and enlisted the help of Dalriada to do so. The two forces met just east of Lough Neagh.
In 637 the settlement of Moira was substantially smaller than it is in present times. However, there was at the very least a motte (the mound of which can still be seen in the village). The area was also much more forrested in the first millenium AD, with the existence of expansive woodland near the hamlet.
Little is known about the actual battle itself. The armies of both Domnall II and Congal were primarily made up of warriors native to Ireland. However Domnall I of Dalriada brought a more varied force to the fight. His army included Scots, Picts, Anglo-Saxons and Britons (Welshmen). At least one side had a substantial cavalry force.
According to Sir Samuel Ferguson "there appears reason to believe that the fight lasted a week", at the end of which the defeated force fled towards the woods of Killultagh. The forces of Ulster and Dalriada were defeated, with Domnall of Dalriada forced to flee north to his kingdom's holdings. Congall was killed in the course of the battle.
The scale of the battle was, however, confirmed in the 19th century when the railway line in Moira was being constructed. Thousands of bodies of men and horses were excavated. When one considers that the survivors probably numbered quite considerably more, then the reputation of the scale of the battle becomes obvious.
With the death of Congal in the battle the chance for Dalaradia and its local allies to undo the advances of Domnall had been scuppered, and the Ulaid had to endure the advances that the High King had made. They were not to be completely subjugated however.
By contrast the consequences were much more keenly felt for Dalriada. The land defeat at Moira was coupled with a naval defeat on the exact same day; at the Battle of the Mull of Kintyre the Ard Rí's fleet had succeeded in defeating Dalriada's. As a result of both battles the High King's forces were able to occupy the Dalriadan lands in north Antrim, unprotected as they now were.
As a direct result of the battle the Uí Néill clan became dominant in the north of Ireland. Their descendants would claim overlordship of at least some of the land until the Flight of the Earls almost a thousand years later in 1607.