It was later expanded to a four battalions and a depot, the legion won distinction in the Walcheren Expedition, the Peninsular War, and the German Campaign of 1813.

It also included Irishmen who had been taken during the 1798 rebellion who were freed during the short peace effected by the Treaty of Amiens on condition of exile, and who had sailed for France in June 1802.

[3] The purpose of the Legion was to align the Irish hearts to the French cause in the imminent invasion of Ireland.

[1] General Pierre Augereau had been ordained to lead the invasion, and wanted Irishmen to serve in his army.

[1] These made a varied group; some were former United Irishmen who were taken prisoner in 1798-99 and then freed during the peace that followed the Treaty of Amiens (1802–03), some had been impressed into the Royal Navy and deserted, and some were German or Polish.

[1] While the Legion was stationed at the Fortress of Mainz in 1806, they were joined by 1,500 Poles[2] and many Irishmen who were sent in 1799 to serve the King of Prussia,[4] Its headquarters was at 's-Hertogenbosch, known to the French as Bois-le-Duc, in what was then the Kingdom of Holland.

[citation needed] In 1808 the Second Battalion fought in the Peninsular War, helping to subdue Madrid during the Dos de Mayo Uprising.

[2] During the battle, Captain John Allen's company's drummer boy continued to beat the charge after having lost both legs, for which he was given the French Legion of Honor.

[1] The Irish Battalions remained on active service with the Army of Portugal through the sieges of Ciudad Rodrigo and Almeida, the Battle of Bussaco and the full course of the advance to the Lines of Torres Vedras.

William Grattan's Adventure of the Connaught Rangers details an engagement between the two units among the rocks above Bussaco.

Solignac's Division took the field once more in the spring of 1811 for the relief of Almeida; at this time the battalion strength was 390 officers and men.

The Irish Regiment made forced marches to arrive on the battlefield of Bautzen during the morning of 21 May, the second day of the battle.

[note 3] Following Macdonald’s retreat after the Battle of the Katzbach with Blucher, General Pothod's Division became isolated from the rest of the army.

A combined Russian and Prussian Army of overwhelming superiority faced the Division but could only attack at the one narrow south-eastern end of the ridge.

[8] In 1805-15 eleven of the regiment's officers were awarded the Legion d'honneur, including Chef de Bataillon Myles Byrne,[9] the colonel, William Lawless, Chef de Bataillon John Tennent, and James Bartholomew Blackwell.

[10] The regiment divided in loyalty during the "Hundred Days", and was officially disbanded by King Louis XVIII on 28 September 1815.