From Lanesborough the regiment marched to Mullingar, of which place its commanding officer, Colonel Brewer, was appointed governor. The quarters of the regiment were infested with parties of armed Roman Catholic peasantry, called rapparees, and on the 28th of April, Colonel Brewer advanced with six hundred men of the Twelfth and eighteenth regiments, and twenty dragoons, towards the castle of Donore, beyond which place two thousand rapparees had taken post and occupied a number of huts. At daybreak the following morning the soldiers arrived at the quarters of the rapparees, who formed for battle on the hills; but when the musketeers of the Twelfth and eighteenth advanced to commence the action, the enemy fled; the soldiers pursued some distance, and killed fifty of the fugitives.
Parties of rapparees continued to hover round Mullingar, and on the 2nd of May, they intercepted a serjeant and four soldiers of the Twelfth regiment between that place and Kinnegad; they put the serjeant and three of the soldiers to death, and put out the eyes of the fourth soldier. Three of the perpetrators of this cruelty were captured; two of them were hanged on the spot, and the third, to save his life, guided Captain Poynes and a hundred soldiers of the regiment, to one of the lurking-places of the rapparees, where the men of the Twelfth fell suddenly upon a large company of these marauders, killed forty, dispersed the remainder, and recovered a quantity of property, which had been taken from the Protestants.
Towards the end of May, one division of the army encamped at Mullingar, where General De Ginkell arrived and assumed the command.
From Mullingar the army advanced to the fort of Ballymore, which was besieged, and surrendered on the 8th of June.
After repairing the breaches of Ballymore, and putting the place in a state of defence, the army advanced to Athlone, and on the 20th of June, the regiment was ordered to support the storming party at the attack of the Westmeath side of the town. Major-General Mackay commanded the troops employed on this service, and after making the necessary arrangements for the attack, took his post on the battery to see the issue, when he observed that the advanced party had missed its way and halted. He instantly hastened to the Twelfth regiment, and taking the first captain he came to by the hand, pointed the way to the breach. The regiment immediately rushed forward, stormed the breach in gallant style, and overcoming the resistance of the Irish, drove them across the bridge to the Connaught side of the town.
Several batteries were raised against the works on the Connaught side of the river, and the grenadier company of the Twelfth was engaged in forcing the passage of the Shannon, and in capturing the town by storm, on the 30th of June, which was a most desperate service, and was performed with distinguished valour and intrepidity.
The Irish army, commanded by a French officer of talent and reputation, General St. Ruth, took up a position near Aghrim, where it was attacked on the 12th of July. During the action, Major-General Mackay ordered the Twelfth, and three other regiments, to pass a difficult bog, ford a rivulet, and drive the Irish from behind the hedges of the nearest enclosures. The soldiers waded through the bog and rivulet, which was waist deep, and drove the Irish out of the first enclosures in gallant style. They afterwards pressed forward with too much ardour, before the troops designed to support them had arrived, and becoming insulated, they were attacked in front and on both flanks by very superior numbers, and driven back to the edge of the bog. The Irish followed, shouting and plying them with musketry; but a support arriving under Major-General Talmash, the four regiments faced about, repulsed their pursuers, and by a spirited effort recovered their lost ground; the cavalry passed the bog near the castle of Aghrim, and by a determined charge completed the overthrow of the Irish army: the French general, St. Ruth, was killed towards the close of the action by a cannon-ball.
The Twelfth regiment had one major, one captain, one ensign, and a number of private soldiers killed, one lieutenant, and seven rank and file wounded.
The regiment afterwards marched with the army to Galway, and formed part of the force employed in the siege of that place, which surrendered on the 21st, and was delivered up on the 26th of July. Major-General Bellasis was appointed governor of Galway, and the Twelfth, twenty-second, and twenty-third regiments were selected to form the garrison of that fortress.
During the remainder of the campaign, the Twelfth regiment was stationed at Galway; and in the autumn, the war in Ireland was terminated by the surrender of Limerick, which delivered that country from the power of King James the Second.