The Battle of the Boyne between King William III and his father - in - law, King James II, was fought on 1 July 1690 ( 11th of July according to our modern calendar ). Both kings commanded their armies in person. William had 36,000 men and James had 25,000, the largest number of troops ever deployed on an Irish battlefield. At stake were the British throne, French dominance in Europe and religious power in Ireland.