The Battle of Newtownbutler takes place on July 31, 1689, near Enniskillen, County Fermanagh. It is part of the Williamite War in Ireland between the forces of William III and Mary II and those of King James II.

In Enniskillen, armed Williamite civilians drawn from the local Protestant population organise a formidable irregular military force. The armed civilians of Enniskillen ignore an order from Robert Lundy that they should fall back to Derry and instead launch guerrilla attacks against the Jacobites. Operating with Enniskillen as a base, they carry out raids against the Jacobite forces in Connacht and Ulster, plundering Trillick, burning Augher Castle, and raiding Clones.

A Jacobite army of about 3,000 men, led by Justin McCarthy, Viscount Mountcashel (in the Jacobite peerage), advance on them from Dublin. Lord Mountcashel’s men consist of three regiments of infantry and two of dragoons. The regiments include his own regiment, Mountcashel (approx. 650 men in 13 companies), The O’Brien regiment (also 13 companies of 650 men), and the Lord Bophin (Burke) regiment. He also has the dragoon regiments of Cotter and Clare, each with seven companies of about 350 dragoons. On July 28, 1689, Mountcashel’s force encamps near Enniskillen and bombards the Williamite outpost of Crom Castle to the southeast of Enniskillen. Crom Castle is almost 20 miles (32 km) from Enniskillen by road and about 5 miles (8.0 km) from Newtownbutler.

Two days later, they are confronted by about 2,000 Williamite ‘Inniskilliniers’ under Colonel Berry, Colonel William Wolseley and Gustave Hamilton. The Jacobite dragoons under Antoine Hamilton stumble into an ambush laid by Berry’s men near Lisnaskea and are routed, taking 230 casualties. Mountcashel manages to drive off Berry’s cavalry with his main force but is then faced with the bulk of the Williamite strength under Wolseley. There is some debate in the sources over troop numbers, though it is believed that Mountcashel has a large number of poorly armed conscripts. Unwisely, Mountcashel halts and draws up his men for battle about a mile south of Newtownbutler.

Williamite histories claim that many of the Jacobite troops flee as the first shots are fired. Up to 1,500 of them are hacked down or drowned in Upper Lough Erne when pursued by the Williamite cavalry. Of the 500 men who try to swim across the Lough, only one survives. Approximately 400 Jacobite officers, along with Lord Mountcashel, the Jacobite commander, are captured and later exchanged for Williamite prisoners, with the other Jacobites being killed. These claims seem unlikely, for several reasons. Each Irish regiment includes approximately 40 officers. The entire force, therefore, would include only about 200 officers. Many of these officers are accounted for in an October 1689 roll call, which shows approximately a 15–20% change in the officer roll call since July for the infantry regiments and 5% for the dragoons. This totals some 20–30 officers in all. Also, the Mountcashel regiment’s roll call for October shows that companies which would normally have 50–60 men, have around 25, which results in a loss of approximately 300–400 men for this regiment. The Cotter and Clare dragoons who ride away from the battle do not have significant losses, based on the October 1689 roll call. Assuming the other two infantry regiments suffer similar losses, gives a total loss of 1,200–1,300. Given their officers are recorded in the October roll and show fewer losses than the Mountcashel regiment among officers, there may be fewer losses in the ranks as well. The Williamite histories acknowledge that they captured approximately 400, including men who are later sent to Derry, which would indicate a total loss of killed, wounded, and missing of 800–900, and likely less. This number is necessarily an estimate based on the available data but should be contrasted with Williamite claims that they killed and drowned 2,000. It appears likely that a couple of hundred men from Mountcashel’s regiment may have fled into the bogs toward Lough Erne, and some of them who made it to the river tried to swim and were drowned, leading to the story of the hundreds drowned.

Lord Mountcashel is wounded by a bullet and narrowly avoids being killed. He later escapes from Enniskillen and returns to lead the Irish Brigade in the French Royal Army. The Jacobite colonel, Sir Thomas Newcomen, 5th Baronet, is killed.

The Williamite victory at Newtownbutler ensures that a landing by Frederick Schomberg, 1st Duke of Schomberg, in County Down in August 1689 is unopposed.

The battle is still commemorated by the Orange Order in Ulster and is mentioned in the traditional unionist song, “The Sash.”

The battle is significant in another way: the regiments on both sides go on to have long and famous histories. On the Williamite side, the Innsikilling Regiment (27th Foot), and on the Jacobite side, the Clare and Mountcashel/Lee/Bulkeley regiments of the Irish Brigade. The two Irish regiments face off again at the Battle of Fontenoy in 1745, where the Irish Brigade famously drives the British army from the battlefield with a charge in the final stage of the battle.