It was therefore resolved that Lord Mountcashel should proceed against Enniskillen from the direction of Dublin, while Berwick and Brigadier Southerland were to approach it from the north and west, and place their commands at his disposal. For this purpose, Berwick was ordered from Derry on the 21st of June. He was to march through Donegal, chastise the outlying insurgents there, and establish his headquarters at Trellick; while Brigadier Southerland, who lay towards Sligo, and under whom Colonel Sarsfield commanded a division of horse, was to move round to Belturbet, and, in his way, scour the country along the south-western side of Lough-Erne. Both were then to drive the enemy within their defences and await the arrival of Mountcashel, who was to proceed from Dublin, through Monaghan and Cavan, when all were to co-operate in a simultaneous movement for the reduction of this rebel stronghold.
On receipt of these orders, Sarsfield, at the head of three troops of horse, one of dragoons, and three battalions of foot,—a force of about five hundred men,—cleared the country along the south-east of the lake, and arrived at Belturbet on the 10th of June. Here he received an order from de Rosen to march forthwith to Omagh, about twenty-five miles north-west of Enniskillen, to protect the Irish besieging army at Derry against rebel attacks from that quarter, and proceeded at once to execute his commission. Southerland, with the remainder of his command—about 1,200 men—advanced through the south of Leitrim, and doubling Lough Oughter, reached the vicinity of Belturbet on the 16th of June. Here he found that Sarsfield had departed for Omagh, and that he was left to cope alone with the united commands of Hamilton, Wolseley, and Lord Blaney. On the 18th, he was informed by one of his spies that the enemy, 15,000 strong, knowing his condition, were about to seize a narrow pass, through which he had advanced, and to attack him in front and rear, with the intention of capturing or annihilating his force before the arrival of Mountcashel.[18]
On receipt of this information, Southerland, leaving Lieutenant-Colonel Scott and two hundred and eighty men in the churchyard of Belturbet to check the pursuit, withdrew in the night, and, by a skilful movement, brought his command in safety to Sligo. The Enniskilleners, baffled in their design, then turned their whole force against Scott, who, after a stubborn contest of two hours, was compelled to surrender: and all the supplies of the garrison, eighty dragoon-horses, seven hundred muskets, and a considerable quantity of gunpowder, fell into the hands of the enemy.
Berwick left Derry on the 21st, and, at the head of his four hundred dragoons, marched rapidly to the town of Donegal, where three hundred of the enemy from Ballyshannon were forming magazines. He approached their position in the night; attacked them at daybreak; killed many, forced the rest to the shelter of the castle; burned the magazines; and marched off with a booty of 1,500 cattle. Being shortly after joined by two regiments of horse and four battalions of foot,[19] which swelled his command to 1,200 men, he advanced, and on the 6th of July formed an encampment at Trellick, about nine miles north-east of Enniskillen.
On the 13th, he advanced with a party to reconnoitre the country and the fortifications of the town, when he was ambushed by a force of two hundred foot and one hundred horse, and attacked with great vigor. But notwithstanding the suddenness of the onset, he turned on them; killed all but six of the infantry; drove the horse within their intrenchments, and returned with a captain, a lieutenant, two pair of colors, and the arms of the slain.
Shortly after this he was raised to the rank of lieutenant-general, and the king ordered that he should have troops and artillery to press the object of the expedition. But de Rosen, whose mission to Ireland seems to have been to disconcert every movement that promised success, again ordered him to Derry, and he abandoned the expedition against Enniskillen with that reluctance which he indicated in after years by the following remark: "It is true, we had few, if any, cannonballs, and scarce any ammunition; but yet, as the Fort of Enniskillen was only a mud fort, we might have carried it; besides, the town being entirely unfortified, we should have got possession of it, and by that means have obliged the fort to surrender." But then it was de Rosen's to command, and Berwick's to obey.
The recall of Berwick left the Enniskilleners again free to renew their excursions and strengthen their fortifications, and they availed themselves abundantly of this temporary advantage. Their forces daily augmented, and they grew more exacting on the country as they increased in power. The garrison of Sligo kept them in check on the western side of the lake, but from Ballyshannon round to Belturbet, a circuit of fifteen miles, all had to quit their homes or yield to their exactions. Their military power towards the end of July was formidable; and, taking the forces of Lord Blaney, Captain Francis Hamilton, Wolseley, and Colonel Creighton (the commandant) into account, must have come up to Southerland's estimate of 15,000 men. Stationed at strong positions around the shores of the Lough; having large depots at Ballyshannon, Enniskillen, and Crom Castle, and acquainted with all the intricacies of the lake and its confluents, they should have been able to cope with an army of twice their number. In addition to this, they had lately received from England ten pieces of cannon, with ball and match to suit; fifty barrels of gunpowder; a large supply of dragoon firelocks and muskets; a corps of engineers and gunners; experienced officers, with commissions to raise new regiments of horse and foot; and eight hundred veterans of Kirke's command, under Colonel Berry.
To drive this force from their network of fortifications, and lay siege to Enniskillen, Mountcashel arrived with about 3,600 men and seven pieces of artillery at Belturbet on the 27th of July.
The town had been abandoned, on his approach, and on the 28th he advanced and invested Crom Castle, on the eastern side. By the 30th he had carried the outer works, and driven the enemy within the walls, though not without considerable loss, and at once opened a cannonade upon the castle. While here, he received word that Colonel Berry was advancing on him by way of Lisnaskea, with eight hundred regulars, followed by the united forces of Wolseley and Hamilton. Without discontinuing the operations against the fort, he withdrew a part of his command about two miles to the eastward, and took post at Newtownbutler. Learning that the enemy's forces had all united, and were too powerful to meet in the open country, he sent Colonel Anthony Hamilton, with O'Brien's regiment of dragoons, to hold them in check, while he himself prepared for a retrograde movement to Belturbet. The troops of Hamilton were drawn into an ambush by Berry, near Lisnaskea; their commander was wounded, his next in command killed, and in a retreat which was ordered, two hundred and thirty were slain or taken prisoners. Mountcashel, on hearing of this disaster, advanced with his own regiment of horse; arrested the retreat and repulsed Berry; but seeing Wolseley, with a force of 8,000 men, close in Berry's rear, he took up his retreat to Belturbet. Berry and Wolseley moved forward rapidly; Mountcashel closely pressed, and considering resistance safer than flight, at last drew up his men about a mile to the south of Newtownbutler, and hastily formed in line of battle.
The action which ensued was disastrous to the Irish army. Opposed by more than double their number, and attacked in front and flank, they fought with great bravery, and the battle might have resulted in their favor, but an unfortunate blunder, in carrying out the general's orders, disarranged their lines, created a panic among the soldiers, and a total rout was the consequence. The lake and its tributaries cut them off from escape in any direction, and, being completely hemmed in, they were slaughtered without mercy. Their loss is estimated to be over 2,000 men, of whom 400 only were killed in the battle, the rest being massacred through the night, to the cry of "No popery!" or drowned in the lake into which they had thrown themselves in the vain hope of escaping the general carnage. Mountcashel himself was wounded and taken prisoner, but was saved from death by a captain named Cooper, to whom he had previously rendered a similar service. Sir Stephen Martin and Lord Abercorn, and many officers of distinction, were killed. All that escaped of this unfortunate command fled towards Belturbet, and, after the capture of their general, the expedition was abandoned.