Hanmer's Regiment had been in Ireland from 1689, when it took part in the relief of the siege of Londonderry. The following year William led his army of 36,000 men south to confront James's 25,000 strong Jacobite Army.
William distracted a large proportion of the Jacobite army by a flank march to the west, leaving the centre, including Hanmer's Regiment free to attack across the Boyne against a lightly held font. Having established a bridgehead the Wiliamite Army drove the Jacobite Army but had to hold out against a series of cavalry counter attacks. The enemy cavalry nearly broke through but were held by the disciplined volleys of the infantry and were finally broken by the arrival of Williams and his cavalry.
A year after raising the siege of Londonderry the Regiment fought its first battle on the south bank of the River Boyne.
.The two kings commanded their respective armies in person. William had 36,000 men made up of soldiers from England, Scotland and Ireland, with Dutch, Danes and Huguenots (French Protestants). William's army, including Hanmer's Regiment, was known as the Williamites. James fielded just 25,000 men (known as Jacobites), who were mainly Irish Catholics, formed from around 6,500 French troops sent to Ireland by King Louis XIV.
The battle opened astride the River Boyne, north of Dublin, near Drogheda, on the road to Belfast. William's camp was on the north side of the river, while James's was on the south bank, with the two armies facing each other. William's plan was to trap the Jacobite army in a pincer movement. He sent 10,000 men of his right flank under Count Meinhard Schomberg and Lieutenant General James Doublas, west towards Slane with the aim of drawing the Jacobites upstream away from the true centre of attack. In response to this display James sent 18,000 men off to the west. With 1,300 Jacobites holding Drogheda, about 6,000 men were left in the centre, at Oldbridge, to confront 26,000 Williamites.
Three hours after marching, Count Meinhard Schomberg crossed the Boyne near Slane and fixed the Jacobite force set to shadow him in battle. Once the enemy were drawn away, with drums beating, the Williamite infantry, including Hanmer's Regiment, in the centre advanced down the side of the valley to the river east of Oldbridge.
The river was up to the soldiers' armpits as they waded across, under musket fire from defending Jacobite infantry, positioned behind hedges and houses. The Jacobites failed to prevent the Williamites, spearheaded by William's Dutch Blue Guards, from gaining a bridgehead on the south bank. Disciplined volleys forced the Jacobites back from the Village of Oldbridge.
At this point the greatly outnumbered Jacobite force launched counter attacks against the Williamite infantry. The infantry were seen off relatively easily but three great cavalry counter attacks though they penetrated around Hanmer's regiment failed to break up the attack. During the battle, little could be seen or heard amidst the smoke and din of battle.
Hanmer's Regiment was forced back into the river but did not break, firing away they drove the enemy back and the Danish infantry crossed further downstream and the bridgehead was secure. William himself crossed downstream at Drybridge with 3,500 cavalry and dragoons to drive off the enemy mounted troops.
Williams attempt to envelope the Jacobites failed. King James's army retreated across the river Nanny at Duleek Casualties totaled about a thousand dead on the Jacobite side, while about 500 Williamites lost their lives. Wounded would have been three or four times these figures. The low death toll was the result of the orderly Jacobite withdrawal and the fact that casualties tended to be inflicted during the pursuit of a defeated army.
However, badly demoralised by their defeat, many of the Irish infantrymen deserted. The Williamites triumphantly marched into Dublin two days later unopposed, as the Jacobite army had abandoned the city and marched to Limerick, behind the river Shannon, where they were besieged.
Even though his army left the field relatively unscathed, James lost his nerve and fled from Ireland with a small escort, returning to exile in France. James's flight from the battlefield and abandonment of the campaign infuriated his Irish supporters, who nick-named him 'James the shit'.
The Regiment's victory on the Boyne represented a landmark in British history, confirming William and Mary firmly on the throne, and consolidated the momentous changes in the manner of British government known as the Glorious Revolution.
In December of the same year a small detachment of the Hanmer's still in Ireland, with similar detachments from two other regiments and some 30 dragoons, found its march opposed by 1,500 of King James's troops. For five hours the little force held its own against continued cavalry charges and sustained musketry fire, and finally forced its passage through these overwhelming numbers and continued its march.