Having given orders for the entire army to take the field, William left Belfast on the 19th of June, 1690, and proceeded to Lisburn and Hillsborough, and while here he authorized the Collector of Customs at Belfast to pay annually £1200 to trustees for the benefit of the dissenting ministers of Down and Antrim, a sum awarded for their loyalty to the royal cause and in compensation for the losses they had sustained thereby. This grant was later inserted in the Civil List, and made payable by the Exchequer. This donation, known as Regium Donum, was increased in 1785 and 1792, and was annually bestowed by the Government on the Presbyterian clergy of Ulster, until the disestablishment of the Church of Ireland.

On the 16th of June James left Dublin to march against his adversary with an army of about 20,000 men, imperfectly disciplined and scantily supplied with even the most necessary requirements for a campaign. He had many brave officers; his French division was composed of first-rate troops, well equipped and well appointed; the Irish horse were admirable, but the dragoons were not so well trained; the Irish infantry consisted for the most part of raw levies, scarcely half armed; and for artillery he was only able to take with him twelve field-pieces with which he had recently been provided by France.

William, who was never so happy as when he was in camp or took the field, was indefatigable in his efforts to raise the spirits of the soldiers and make them forget the calamities of the past year. His forces were ordered to assemble at Loughbrickland, and to prepare for an immediate advance, for he was fully determined to take the earliest opportunity to fight. When Schomberg and several other officers recommended caution and delay, he answered decisively: "I came not to Ireland to let grass grow under my feet". He joined the army at Loughbrickland on the 22nd of June, and immediately ordered the troops to change camp, with a view to make their march to new ground partake of the nature of a review. The day was windy and dusty, and some of his officers thought that the King, who suffered from weak lungs, would review the troops from a distance and thus escape the dust raised by their march. William, however, did not content himself with a perfunctory inspection, but rode in among the regiments, inspected the troops severally as well as collectively with a critical eye, and, with a few grave words addressed to each regiment, greatly raised the spirits of his men. He then took up his residence in the camp in a little wooden structure specially designed for the purpose by Wren, and the men, elated by having the King in their midst, boasted that once encamped amongst them their leader never left them until he left the country.

In camp the King lived a life of Spartan simplicity, refusing to sign an order for wine, saying he would drink water as his soldiers did. From early morning until late in the day he was in the saddle inspecting his men, or taking the lie of the country so as to be prepared for action at any moment. There was a healthy activity in his movements which affected his men for good, and led to a wholesome spirit of emulation by which the entire army of 36,000 men greatly benefited. The King, in addition, wisely caused the fleet to accompany the southward movement of the army, hugging the coast the while. Thus from time to time it was visible to the troops, to whom it proved the source of not a little satisfaction.

James advanced to Dundalk, while William was encamped a few miles beyond Newry; and, in order to ascertain the strength of his opponent, James dispatched, on the 22nd of June, Colonel Dempsey, with sixty horse, and Lieutenant-Colonel FitzGerald, with a party of grenadiers, to lie in wait for one of William's reconnoitring-parties. This duty was so well performed by the Jacobite forces that a detachment of between two and three hundred of William's foot and dragoons was routed with great loss at the half-way bridge between Dundalk and Newry. An English officer, who was taken prisoner, represented William's army as 50,000 strong; and although this was supposed by James to be a gross exaggeration, intended to have the effect of inducing him to fly, he nevertheless wrote privately to Sir Patrick Trant, Commissioner of Inland Revenue, ordering him to have a vessel at Waterford ready to convey him to France in case of disaster, and he even sent his luggage to Waterford to be put on board. James now commenced his retrograde movement and retired to Ardee. When, therefore, William's advanced guard reached Dundalk, nothing was to be seen of the Jacobite army save a great cloud of dust which was slowly rolling southwards. The Jacobites retreated by easy marches, and on the 28th commenced recrossing the Boyne, on the right bank of which James resolved to make a stand, because, as he tells us in his Memoirs, had he left the passage open to William, he would have been obliged to abandon all Leinster to him.

William continued to push forward till, on Monday morning, the 30th of June, 1690, his army, marching in three columns, came within two miles of Drogheda, and, at about nine o'clock, within sight of the enemy's camp. Here the King, who rode at the head of his advanced guard, observed a hill to the west of the town, near the southern frontier of the County Louth, and to the summit of this he went with some of his principal officers to obtain a fuller view of the Jacobite position. This had been well chosen, on ground which sloped down to the river, and backed by an amphitheatre of hills. The Jacobite army was encamped on the declivity of the hill of Donore, with its right wing towards Drogheda, and its left extending in two lines up the river to a morass which was difficult to pass. As there are no considerable inequalities in the surface, the whole of James's lines must have been visible from the heights on the opposite side of the river, and to a great extent exposed to the fire of William's artillery. James's centre was at the hamlet of Oldbridge, close to the bank of the river, where he caused some entrenchments to be hastily thrown up to defend the principal fords, of which there are four near this point, a fifth being a little lower down the stream, and two or three others a few miles higher up in the direction of Slane. There are two islands in the river near Oldbridge which facilitate the passage; and at that season—which was remarkable for drought—and at the time of low water the Boyne was fordable throughout a great part of its course. James himself took up his position at a small ruined church on the top of the hill of Donore, three miles behind which lay the village and pass of Duleek, which afforded him the only means of retreat in case of defeat, and which was so narrow that it could be defended with ease. Drogheda, which was garrisoned by Jacobites, was at that time only a knot of narrow, crooked lanes encircled by a ditch and a mound. The houses were built of wood, with high gables and projecting upper storeys.

While his army marched into camp, William, desirous to gauge more closely the strength of the Jacobite army, rode with some of his officers within musket-shot of the river, opposite one of the fords. Having conferred here for some time with them, the King continued his course westward, and after a while he seated himself on the grass on a piece of rising ground in order to take some refreshment. The Jacobite generals, Berwick, Sarsfield, and Tyrconnell, had observed his movements as they rode slowly along the opposite bank of the river, and they noted the spot where William had seated himself. They now ordered two field-pieces, concealed by a party of horse, to be brought into a ploughed field opposite William's resting-place, and trained from behind a hedge on the King and his attendants. When, after being seated for about an hour, William rose and was in the act of mounting his horse, both guns were discharged, one killing a man and two horses in a line with the King, but at a little distance from him ; the other grazed William's right shoulder, tore his coat, and slightly wounded him. This naturally caused some commotion in the immediate surroundings of the royal party, which resulted in a report in the Jacobite camp that the Prince of Orange was killed, a report which even penetrated via Dublin to Paris before it was contradicted. The two guns continued to fire on the party of horse which attended William's movements, until he directed them to retire under shelter of the hill, while he himself had his wound dressed; which being done, he remounted and rode through the camp to assure the army of his safety.

That army consisted of some 36,000 men of various nationalities, the composite parts of which it was composed being so admirably summarized by Macaulay that his description of it may here be given: "About half the troops were natives of England. Ormonde was there with the Life Guards, and Oxford with the Blues. Sir John Lanier, an officer who had acquired military experience on the Continent, and whose prudence was held in high esteem, was at the head of the Queen's regiment of horse. . . . There were Beaumont's foot, who had in defiance of the mandate of James, refused to admit Irish Papists among them, and Hasting's foot, who had, on the disastrous day of Killiecrankie, maintained the military reputation of the Saxon race. There were the two Tangier battalions, hitherto known only by deeds of violence and rapine. . . . Two fine English regiments, which had been in the service of the States General, and had often looked death in the face under William's leading, followed him in this campaign, not only as their general, but as their native King. . . . The former was led by an officer who had no skill in the higher parts of military science, but whom the whole army allowed to be the bravest of all the brave, John Cutts. The Scotch footguards marched under the command of their countryman James Douglas. Conspicuous among the Dutch troops were Portland's and Ginkell's Horse, and Solmes's Blue regiment, consisting of two thousand of the finest infantry in Europe. ... A strong brigade of Danish mercenaries was commanded by Duke Charles Frederic of Wurtemberg. . . . Among the foreign auxiliaries were a Brandenburg regiment and a Finland regiment. . . . Mit-chelburne was there with the stubborn defenders of Londonderry and Wolseley with the warriors who had raised the unanimous shout of 'Advance' on the day of Newton Butler. Sir Albert Conyngham . . . had brought from the neighbourhood of Lough Erne a regiment of dragoons which still glories in the name of Enniskillen."