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An article from History Ireland magazine gives a good account of the battle and the eventual collapse of the Jacobite army.
Importantly, prior to the battle,
Saint-Ruhe bolstered his army’s morale, creating a personality cult around himself with bloodthirsty promises of what the army would do to its foes
So the morale of the army was anchored on Saint-Ruhe with the consequence that his death had the potential to be taken very badly by the Jacobite forces.
Immediately before Saint-Ruhe's death, things were looking fairly good for the Jacobites:
Saint-Ruhe recognised this risk to his left and took personal charge of the Life Guards from Sarsfield’s wing with the aim of crushing the threat. When he was killed, the Life Guards kept his death secret (because of the risk to morale) which exposed another problem with the battle planning:
[Keeping Saint-Ruhe's death a secret] was a disastrous decision: Sheldon’s men, awaiting Saint-Ruhe’s orders, became ever more nervous as the general failed to appear. Sheldon would not order a charge as the enemy cavalry were in Dorrington’s area of responsibility and he, Sheldon, needed Saint-Ruhe’s authority to order his men there. Worse still, Saint-Ruhe had committed one of the greatest sins a commander can commit: he had not divulged his battle strategy to his senior officers. Not even Sarsfield knew the plan. Sheldon waited in vain for his general and orders.
This effectively paralyzed the defence of the Jacobite left and allowed the vulnerable Williamite cavalry to take control of that part of the battlefield. The Jacobite dragoons who had been only opposition to the Williamite cavalry were increasingly outnumbered and their commander lead them off the field of battle. This action exposed the left flank of the Jacobite army to attack by the Williamite cavalry.
The Irish defences had been designed to protect against a frontal, not a flank, assault, and the cavalry arrived so suddenly that there was no time to change positions, nor did the training and experience of the Irish infantry allow battalions to form platoons or squares to meet the cavalry, even had there been time. Cavalry charged into the infantry who ran for the hilltop as the hors*m*n penetrated even further.
As often happens when morale breaks in an army, without a strong leader to re-take control; cohesion disappears, panic spreads, and it becomes every man for himself. Of the Jacobite commanders, only Sarsfield kept any control of his men but that wasn't going to be enough to save the day, he merely bought time for some of the Jacobite forces to slip away.