The Irish Brigade is a brigade in the French Royal Army composed of Irish exiles, led by Lord Mountcashel. It is formed in May 1690 when the five regiments sent from Ireland arrive in France in exchange for a larger force of King Louis XIV‘s well-trained French infantry who are sent to fight in the Williamite War in Ireland. The regiments comprising the Irish Brigade retain their special status as foreign units in the French Army until nationalised in 1791.
King Louis XIV wants to support James II in his quest to regain the British crown from William of Orange, but he can ill-afford the loss of 6,000 soldiers during his own struggle with William on the continent. Louis demands Irish replacements, ill-trained though they might be, in exchange. The Irish regiments sail out on the same ships that landed the French troops under Count de Lauzun.
Soon after arriving in France, the five regiments are reorganized into three, commanded by Lord Mountcashel, Daniel O’Brien, and Theobald Dillon, whose family continues in command of this regiment for a one hundred years. Mountcashel commands this first Irish Brigade which is known as Lord Mountcashel’s Irish Brigade. He has grown up in France, and become fluent in the French tongue after his father had lost everything due to his participation in the fight against Oliver Cromwell and subsequent exile to France. Mountcashel’s brigade is joined by Patrick Sarsfield‘s men in late 1691. The Irish Brigade carries on in French service for 100 years and amass a record equaled by few military organizations in history.
Like Sarsfield, Mountcashel does not survive for very long in French service. Very shortly after his arrival in France, on September 11, 1690, he is seriously wounded in the chest fighting in Savoy near Mountiers de Tarentaise. Although he recovers from this wound and continues to command the brigade, the wound continues to hamper him. In 1694, he leaves the brigade and seeks relief from his wounds in the baths at Baréges in the Pyrenees. Unfortunately, Justin McCarthy, Lord Mountcashel, dies there on July 1, just short of a year after Patrick Sarsfield is killed at the Battle of Landen.
The Brigade ceases to exist as a separate and distinct entity on July 21, 1791. Along with the other non-Swiss foreign units, the Irish regiments undergo “nationalization” at the orders of the National Assembly. This involves their being assimilated into the regular French Army as line infantry, losing their traditional titles, practices, regulations and uniforms.