The first Irish troops to serve as a unit for a continental power formed an Irish regiment in the Spanish army of Flanders in the Eighty Years' War in the 1580s. The regiment had been raised by an English Catholic, William Stanley, in Ireland from native Irish soldiers and mercenaries, whom the English authorities wanted out of the country. (See also Tudor re-conquest of Ireland) Stanley was given a commission by Elizabeth I and was intended to lead his regiment on the English side, in support of the Dutch United Provinces. However, in 1585, motivated by religious factors and bribes offered by the Spaniards, Stanley defected to the Spanish side with the regiment. In 1598 Diego Brochero de Anaya wrote the Spanish King Philip III:
"that every year Your Highness should order to recruit in Ireland some Irish soldiers, who are people tough and strong, and nor the cold weather or bad food could kill them easily as they would with the Spanish, as in their island, which is much colder than this one, they are almost naked, they sleep on the floor and eat oats bread, meat and water, without drinking any wine.The unit fought in the Netherlands until 1600 when it was disbanded due to heavy wastage through combat and sickness.
In 1607 the "Flight of the Earls" occurred, with the defeat of the rebels of the Nine Years' War. The Earl of Tyrone Hugh O'Neill, the Earl of Tyrconnell Rory O'Donnell and the Lord of Beare and Bantry, Donal O'Sullivan, along with many chiefs and their followers from Ulster, fled Ireland. They hoped to get Spanish help in order to restart their rebellion in Ireland, but King Philip III of Spain did not want a resumption of war with England and refused their request.
Nevertheless, their arrival led to the formation of a new Irish regiment in Flanders, officered by Gaelic Irish nobles and recruited from their followers and dependents in Ireland. This regiment was more overtly political than its predecessor in Spanish service and was militantly hostile to the English Protestant government in Ireland. The regiment was led by Hugh O'Neill's son John. Prominent officers included Owen Roe O'Neill and Hugh Dubh O'Neill.
A fresh source of recruits came in the early seventeenth century, when Roman Catholics were banned from military and political office in Ireland. As a result, the Irish units in the Spanish service began attracting Catholic Old English officers such as Thomas Preston and Garret Barry. These men had more pro-English views than their Gaelic counterparts and considerable animosity was created over plans to use the Irish regiment to invade Ireland in 1627. The regiment was garrisoned in Brussels during the truce in the Eighty Years' War from 1609-1621 and developed close links with Irish Catholic clergy based in the seminary there, creating the famous Irish Colleges — most notably, Florence Conroy.
Many of the Irish troops in Spanish service returned to Ireland after the Irish Rebellion of 1641 and fought in the armies of Confederate Ireland - a movement of Irish Catholics. When the Confederates were defeated and Ireland occupied after the Cromwellian conquest of Ireland, around 34,000 Irish Confederate troops fled the country to seek service in Spain. Some of them later deserted or defected to French service, where the conditions were deemed better. At the time of the Napoleonic Wars there were still three Irish infantry regiments in the Spanish army: Irlanda (raised 1698); Hibernia (1709); and Ultonia (1709). However in the later years of the existence of these units only the officers were Irish or of Irish descent, the men being predominantly Spanish or other foreigners. All three regiments were finally disbanded in 1815.
France recruited many foreign soldiers; Germans, Italians, Walloons and Swiss. André Corvisier, the authority on French military archives, estimates that foreigners accounted for around 12% of all French troops in peacetime and 20% of troops during warfare. The Irish regiments earned more than their French counterparts and wore the redcoat of the British army.
The crucial turning point came during the Williamite War in Ireland (1688-91), when Louis XIV gave military and financial aid to the Irish Jacobites. In return for 6000 French troops, Louis demanded 6000 Irish recruits for use in the Nine Years War against the Dutch. These men, led by Justin McCarthy, Viscount Mountcashel formed the nucleus of the French Irish Brigade.
Later, when the Irish Jacobites under Patrick Sarsfield surrendered at the Treaty of Limerick, they were allowed to leave Ireland for service in the French Army. Sarsfield's "exodus" included 14,000 soldiers and 10,000 women and children. This is popularly known in Ireland as the "Flight of the Wild Geese".
Initially, these units were not integrated into the French Army, but were assigned to the court in exile of James II, deposed in the Glorious Revolution, whom Louis deemed the legitimate King of England, Ireland and Scotland. They were later incorporated into the Irish Brigade of the French Army.
Like the earlier Irish units in Spanish service, the French Irish regiments were quite politicised, being composed of dispossessed Irish Catholics, who were committed to a Stuart restoration in Britain and Ireland. Famously, the Irish Brigade distinguished themselves in the Battle of Fontenoy against British troops in 1745.
Up until 1745, Catholic Irish gentry were allowed to recruit soldiers for France in Ireland. The authorities in Ireland saw this as preferable to the potentially disruptive effects of having large numbers of unemployed young Catholic men of military age in the country. However, after a composite Irish detachment from the French Army (drawn from each of the regiments comprising the Irish Brigade and designated as "Irish Picquets") was used to support the Jacobite Rising of 1745 in Scotland, the British realised the dangers of this policy and banned recruitment for foreign armies in Ireland. After this point, the rank and file of the Irish units in French service were increasingly non-Irish although the officers continued to be recruited from Ireland.
During the Seven Years War efforts were made to find recruits from amongst Irish prisoners of war or deserters from the British Army. Otherwise, recruitment was limited to a trickle of Irish volunteers who were able to make their own way to France, or from the sons of former members of the Irish Brigade who had remained in France. During the Seven Years War the Irish Regiments in French service were: Bulkeley, Clare, Dillon, Rooth, Berwich and Lally. Additionally, there was a regiment of cavalry, Fitz James. By the end of the 18th century even the officers of the Irish Regiments were drawn from Franco-Irish families who had settled in France for several generations. While often French in all but name such families proudly retained their Irish heritages.
Following the outbreak of the French Revolution the Irish Brigade ceased to exist as a separate entity on 21 July 1791 when the 12 non-Swiss foreign regiments then in existence were integrated into the line infantry of the French Army, losing their distinctive status, titles and uniforms. Many left the service in 1792 when Louis XVI was deposed, as their oath of loyalty was to him and not to the French people. Napoleon Bonaparte subsequently raised a small Irish unit composed of veterans of the Irish Rebellion of 1798.
"I cannot but highly esteem those gentlemen of Ireland who, with all the disadvantages of being exiles and strangers, have been able to distinguish themselves by their valour and conduct in so many parts of Europe, I think, above all other nations.It was some time before the British armed forces began to tap into Irish Catholic manpower. In the late eighteenth century, the Penal Laws were gradually relaxed and in the 1790s the laws prohibiting Catholics bearing arms were abolished.
Thereafter, the British began recruiting Irish regiments for the Crown Forces — including such famous units as the Connaught Rangers. It has been estimated that up to forty percent of Arthur Wellesley, 1st Duke of Wellington's army in the Peninsular War was Irish. By 1830 over 42% of the British army was Irish (41% were English). Several more Irish units were created in the 19th century. By 1914 specifically Irish infantry regiments in the British Army comprised the Prince of Wales's Leinster Regiment, the Royal Dublin Fusiliers, the Irish Guards, the Royal Irish Regiment, the Royal Inniskilling Fusiliers, the Royal Irish Rifles, the Royal Irish Fusiliers, the Connaught Rangers and the Royal Munster Fusiliers. With the creation of the Irish Free State in 1922 five of the above regiments were disbanded, with most of the remainder undergoing a series of amalgamations between 1968 and 2006. The United Kingdom still retains three Irish regiments: the Irish Guards, the Royal Irish Regiment, and the London Irish Rifles.