It was a fight between protestant William of Orange, who had been crowned king of England, Scotland and Ireland during the 'Glorious Revolution' the year before, and his uncle, father-in-law and the deposed monarch James II, who was a catholic.
James had fled to France after losing his crown in 1688. He was encouraged by King Louis XIV to take back the monarchy, and the French king backed James with thousands of troops. James headed to Ireland in 1689 and was welcomed by the largely Catholic population.
William landed in Ireland in June the following year. He brought with him a huge army and had around 36,000 men ready to fight.
William marched towards Dublin and was met by James' force of 24,000 men at the Boyne. This was the last time two crowned kings of England, Scotland and Ireland would ever meet in battle.
The two sides fought to a stalemate. William's army was bigger and better trained, but his troops were prevented from crossing the river by constant charges from James' cavalry.
A quarter of William's troops were sent to cross the river at Roughgrange, with James sending around two thirds of his men and most of his artillery to hold them off. Both sides had failed to realise that the ground here was impassable, and all these soldiers effectively sat out the battle, weakening James' ability to defend other parts of his line.
To break the deadlock, William lead a crossing of his own horsemen. He was a frail man and had been wounded by a cannon ball. As he led his cavalry across the river his horse got stuck and William suffered an asthma attack. One of his troops spotted the king was in trouble and carried him to safety.
But two thousand of his cavalry did make it it across the Boyne and now they were able to attack the Jacobites directly.
The Jacobites lines were thinly stretched and the infantry were poorly equipped. Many were farmers who'd received little training and were armed with tools from their farms. Realising they were being overwhelmed, the Jacobites headed to the high ground of the hill of Donore.
Chased by the Williamites, the catholic troops made it to the church on top of the hill (the ruins are still there today). The protestant army approached on three sides and fierce hand-to-hand fighting took place.
William rejoined the battle and, after hours of fighting, his superior numbers told. The remaining Jacobites fled and the battle was won.
Unlike William, who was in the thick of battle, James had stationed himself several miles away. Realising his forces were defeated, he fled and made his way back to France. This act of cowardice angered his supporters in Ireland who gave him the nickname, James the Shit.
He would never return to the kingdoms he hoped to rule again.
Book the tour: The War of Three Kings in Ireland takes you to the battlefield of the Boyne and hill at Donore, where you can relive the drama and terror of the troops who fought there. You'll be led by Barry Hilton, and expert on the Williamite-Jacobite wars in Ireland.
For Three Kings in Ireland tour dates and itinerary details
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Picture credit: The Battle of the Boyne by Jan van Huchtenburgh, available under the Wiki Creative Commons license