The Irish Brigade (Irish: Briogáid Éireannach, French: Brigade irlandaise) was a brigade in the French Royal Army composed of Irish exiles, led by Lord Mountcashel. It was formed in May 1690 when five Jacobite regiments were sent from Ireland to France in exchange for a larger force of French infantry who were sent to fight in the Williamite War in Ireland. The regiments comprising the Irish Brigade retained their special status as foreign units in the French Army until nationalised in 1791.

When King James II went to Ireland in March 1689, Ireland was ruled by his viceroy Tyrconnell and was held by the Irish Army, which was loyal to King James. There seemed to be no need for the deployment of French troops in Ireland and Louis XIV needed his troops elsewhere during the Nine Years' War. When the Irish Army showed its weakness by failing to win the Siege of Derry and losing the minor Battle of Newtownbutler on 31 July 1689, Lauzun was sent to Ireland with a French force of 5000 men but Ireland had to send Irish troops to France in exchange. This was the Irish Brigade, formed in May 1690. It consisted of five regiments, comprising together about 5000 men. The regiments were named after their colonels:

The French reformed them and disbanded Butler's and Feilding's, incorporating their men into the remaining three regiments, which were:

These three regiments formed the first Irish Brigade in France and were known as Lord Mountcashel's Irish Brigade and served the French with distinction during the remainder of the Nine Years' War (1689–97).

Under the terms of the Treaty of Limerick in 1691, which ended the war between King James II and VII and King William III in Ireland, a separate force of 12,000 Jacobites of the Irish Army had arrived in France in an event known as Flight of the Wild Geese. These were kept separate from the Irish Brigade and were formed into King James's own army in exile, albeit in the pay of France. Dorrington's regiment, later Rooth or Roth, following the Treaty of Ryswick in 1698, was formed from the former 1st and 2nd battalions James II's Royal Irish Foot Guards (formerly on the Irish establishment) of Britain.

With the Treaty of Ryswick in 1697, King James's army in exile was disbanded, though many of its officers and men were reformed into new regiments. Having been merged into the original Irish Brigade these units served the French well until the French Revolution. Other Irishmen – such as Peter Lacy – proceeded to enter the Austrian service on an individual basis.

The Irish Brigade became one of the elite units of the French Army. While increasingly diluted by French and foreign recruits from elsewhere in Europe, its Irish-born officers and men often aspired to return to aid Ireland and regain their ancestral lands, as some did during the Jacobite Rebellion of 1745.

Irish regiments participated in most of the major land battles fought by the French between 1690 and 1789, particularly Steenkirk (1692), Neerwinden (1693), Marsaglia (1693), Blenheim (1704), Almansa (1707), Malplaquet (1709), Fontenoy (1745), Battle of Lauffeld (1747); and Rossbach (1757).