No date in Irish history is better known than 1690. No Irish battle is more famous than William Ill's victory over James II at the River Boyne, a few miles west of Drogheda. James, a Roman Catholic, had lost the throne of England in the bloodless "Glorious Revolution" of 1688. William was Prince of Orange, a Dutch-speaking Protestant married to James's daughter Mary, and became king at the request of parliament. James sought refuge with his old ally, Louis XIV of France, who saw an opportunity to strike at William through Ireland. He provided French officers and arms for James, who landed at Kinsale in March 1689. The lord deputy, the Earl of Tyrconnell, was a Catholic loyal to James, and his Irish army controlled most of the island. James quickly summoned a parliament, largely Catholic, which proceeded to repeal the legislation under which Protestant settlers had acquired land.

During the rule of Tyrconnell, the first Catholic viceroy since the Reformation, Protestants had seen their influence eroded in the army, in the courts and in civil government. Only in Ulster did they offer effective resistance. In September 1688, while James was still king, apprentice boys in Londonderry closed the city's gates to deny admission to a Catholic regiment under Lord Antrim. In April 1689, the city refused to surrender to James's army, and survived the hardships of a three-month siege before relief came by sea. The Protestants of Enniskillen defended their walled city with equal vigour, and won a number of victories over Catholic troops. Eventually, James withdrew from the northern province.

William could not ignore the threat from Ireland. In August 1689 Marshal Schomberg landed at Bangor with 20,000 troops and, with Ulster secure, pushed south as far as Dundalk. James's army blocked further progress towards Dublin, but there was no battle and the two armies withdrew to winter quarters. In March 1690 the Jacobite army was strengthened by 7,000 French regulars, but Louis demanded over 5,000 Irish troops in return. The Williamites were reinforced by Danish mercenaries and by English and Dutch regiments. When William himself landed at Carrickfergus on 14 June, he was able to muster an army of 36,000 men. He began the march towards Dublin. There was some resistance near Newry, but the Jacobites soon withdrew to the south bank of the River Boyne.

The battle was fought on 1 July 1690 at a fordable river bend four miles west of Drogheda. The main body of Williamite infantry was concentrated on fording the river at the village of Oldbridge, which was approached by a deep and sheltering glen. First, however, a detachment of cavalry and infantry made a flanking attack upstream, which forced James to divert troops to prevent his retreat being cut off. William's army was stronger by at least 10,000 men, but after these troops were drawn off he had three-to-one superiority in the main arena. By mid-aftemoon the Jacobite army was in retreat, outpaced by James himself, who rode to Dublin to warn the city of William's approach. He was in France before the month was out. On 6 July William entered Dublin, where he gave thanks for victory in Christ Church Cathedral.

The Battle of the Boyne is recalled each July in the celebrations of the Orange Order, not on the first day but on "the Twelfth", for eleven days were lost with the change from the Julian to the Gregorian calendar in 1752. It was not the end of the Williamite campaign, and the King had returned to England before the Dutch general Ginkel's victory at Aughrim and the final Irish surrender after the siege of Limerick in 1691. The Treaty of Limerick was not ungenerous to the defeated Catholics, but they were soon to suffer from penal laws designed to reinforce Protestant ascendancy throughout Irish life.