On August 31, 1803, the government of the French Republic proposed to the prisoners of war in its custody to form a legion to be called the “Irish Legion”, intended mainly to fight the English troops.
Its quarters were established in Landau and it was organized as a Light Infantry composed of a battalion of 5 companies. It served first in Prussia and then in Poland.
Around 1807, it included 1 rifle company, 1 company of aerobatics and 4 companies of fighters. It took part in the beginning of the Spanish campaign. The Emperor Napoleon visited it during his stay in Burgos.
In 1809, 2 marching battalions and 1 reserve battalion were formed in Landau. Recruitment was done among foreigners (the French were excluded).
In 1810, the first battalion served in Vlissingen in Holland, the 2nd and 3rd in Spain and the 4th in Landau.
In 1811, by a decree of August 11, it was attached to the 3rd Foreign Regiment.