While James was holding his court at Dublin, in May 1689 he received intelligence that the French fleet, commanded by the Count of Chateau Renaud, had anchored in Bantry Bay and had put on shore a large quantity of military stores and a supply of money. William’s Admiral, Arthur Herbert, who had just been sent to Irish waters with an English squadron for the purpose of intercepting the communications between Brittany and Ireland, learned where the enemy lay, and sailed into Bantry bay with the intention of giving battle. But the wind was unfavourable to him and his force was inferior to the French fleet; and after some firing, which caused no serious loss to either side, he thought it prudent to stand out to sea, while the French retired into the recesses of the harbour. Herbert steered for Scilly, where he expected to find reinforcements; and Chateau Renaud, content with the credit which he had acquired, and afraid of losing it if he stayed, hastened back to Brest, though earnestly entreated by James to come round to Dublin.
King William did not waste a lot of time; he landed at Carrickfergus on 14 June 1690, assembled his army and marched for Dublin, dealing with James’ army at the Battle of the Boyne on 1 July and entering Dublin a few days later. He left the subsequent ‘mopping-up’ to his generals in order to return to England, and from there to the main war with France in Holland. Mopping-up took until 3 October 1691 when the peace of Limerick was signed. It also took a great many more lives than those that had been lost at the Boyne.
The Despard Diaries state that a William Despard was a Colonel of Engineers under King William at the Boyne, but I have found no record of this. My basis for this is that: none of the accounts that I have read of the battle mention his presence; the extensive army lists drawn up in advance of William’s expedition do not include a Despard (nor a Col. Kendall although a Col Kendall had a lease of a property in Westminster in 1699-1701); William of Orange’s Col. of Engineers was called Francis Philiponneau de la Motte (appt. 15.5.1690 and on 6.4.1692 Chief Engineer of Ireland at £1,000pa – his brother Jean was naturalised by petition to the House of Lords – called the Seiur de Montargier). King James had Viscount Mountjoy (Sir William Stewart, who married Mary Coote) as head of his artillery. He was a Protestant and James was unsure of his loyalty, so he sent him on a diplomatic mission to Paris with secret instructions that he be detained there. James then appointed Justin MacCarthy in his place.