From October 1690 until May 1691 no military operation on a large scale was attempted in the Kingdom of Ireland. During that winter and the following spring the island was divided almost equally between the contending parties. The whole of Ulster, the greater part of Leinster, and about one third of Munster were now controlled by the Williamites; the whole of Connaught, the greater part of Munster and two or three counties of Leinster were still held by the Jacobites.
Continuous guerrilla activity persisted, however, along the rough line of demarcation. In the spring of 1691, James’s Lord Lieutenant, Tyrconnell, returned to Ireland, followed by the distinguished French general Saint Ruth, who was commissioned as Commander-in-Chief of the Jacobite army. Saint Ruth was a man of great courage and resolution but his name was synonymous with the merciless suppression and torture of the Protestants of France, including those of the district of Orange in the South, of which William was Prince.
The Marquess of Ruvigny, hereditary leader of the French Protestants, and elder brother of that brave Caillemot who had fallen at the Boyne, now joined the Dutch general Ginkell, who was strengthening the Williamite army at Mullingar. Ginkell first took Ballymore where he was joined by the Danish auxiliaries under the command of the Duke of Wurtemburg, and then the strategic town of Athlone.
Thus was the stage set for one of the fiercest battles of that age or any other. Determined to stake everything in a final showdown St Ruth pitched his camp about thirty miles from Athlone on the road to Galway. He waited for Ginkell on the slope of a hill almost surrounded by red bog, chosen with great judgement near the ruined castle of Aughrim.
Soon after 6 o’clock on the morning of 12 July, 1691, the Williamite army moved slowly towards the Jacobite positions. Delay was caused, however, by a thick fog which hung until noon and only later in the afternoon did the two armies confront each other.
The Jacobite army of twenty-five thousand men had further protected themselves with a breastwork constructed without difficulty. The Williamites, numbering under twenty thousand, advanced over treacherous and uneven ground, sinking deep in mud at every step. The Jacobites defended the breastwork with great resolution for two hours so that, as evening was fast closing in, Ginkell began to consider a retreat. St Ruth was jubilant and pressed his advantage.
However, Ruvigny and Mackay, with the Huguenot and British Cavalry, succeeded in bypassing the bog at a place where only two horsemen could ride abreast. There they laid hurdles on the soft ground to create a broader and safer path and, as reinforcements rapidly joined them, the flank of the Jacobite army was soon turned. St Ruth was rushing to the rescue when a cannonball took off his head. He was carried in secret from the field and, without direction, the Jacobites faltered. The Williamite infantry returned to their frontal attack with rugged determination and soon the breastwork was carried. The Jacobites retreated fighting bravely from enclosure to enclosure until finally they broke and fled.
This time there was no William to restrain the soldiers. Only four hundred prisoners were taken and not less than seven thousand Jacobites were killed, a greater number of men in proportion to those engaged than in any other battle of that time. Of the victors six hundred were killed, and about a thousand were wounded. If the night had not been moonless and visibility reduced by a misty rain, which allowed Sarsfield to cover the retreat, scarcely a Jacobite would have escaped alive.
Waiting in the wings with his own army was a remarkable character named Balldearg O’Donnell. He had arrived from Spain shortly after the Battle of the Boyne claiming to be a lineal descendant of the ancient ”Gaelic” Cruthin kings of Tyrconnell in Ulster. He also claimed to be the O’Donnell ‘with a red mark’ (ball dearg) who, according to ancient prophecy, was destined to lead his followers to victory. Many ordinary Ulster Catholics had flocked to his standard, causing great hostility on the part of Tyrconnell who saw him as a threat to his own earldom.
Balldearg thus remained aloof from the Battle of Aughrim. He proceeded to join the standard of William with 1200 men on 9th September, 1691, and marched to assist in the reduction of the Jacobite town of Sligo. This garrison surrendered on 16th September, 1691, on condition that they were conveyed to Limerick. Balldearg remained loyal to William and later entered his service in Flanders, with those of his men who elected to follow him.