I believe that this song dates from the 1st Carlist War, a civil war in Spain (1833 to 1839), fought between supporters of the regent, Maria Christina, acting for Queen Isabella II of Spain, and those of the late king's brother, Carlos de Borbón. The Carlists supported return to an absolute monarchy.
The British Auxiliary Legion was raised and sent to Spain to support the Liberals and Queen Isabella against the Carlists (headed by 'the King of Spain'). It was raised mainly in the ports and cities of Britain and Ireland in 1835 and fought in the Basque territory in north-eastern Spain. The Legion was funded and the soldiers paid by the Spanish crown. In the summer of 1836 a force of 10,000 men under the command of General Sir George De Lacy Evans had assembled in San Sebastian. They fought near Hernani and Vittoria. The fighting was savage; no quarter was given. A former soldier wrote that to fall into the enemy's hands was certainly a tortured death. The volunteers signed up for 2 years service and a great many were Irishmen.
One of the Legion's two cavalry regiments was titled:
       2nd Queen's Own Irish Lancers – (predominantly Irishmen).
The Legion's ten battalions of infantry were organised into "English", "Scottish" and "Irish" brigades.
       1st English Battalion
       2nd English Battalion
       3rd Westminster Grenadiers – English
       4th Queen's Own Fusiliers – English
       5th Scotch – Scottish
       6th Scotch Grenadiers – Scottish
       7th Irish Light Infantry – Irish
       8th Highlanders – Scottish
       9th Irish Grenadiers – Irish
       10th Munster Light Infantry – Irish

The 7th Irish Light Infantry, 9th Battalion (Irish Grenadiers) and 10th battalion (Munster Light Infantry) were brigaded together under Brigadier-General Charles Shaw, a veteran of 1815 and the Portuguese Civil War. The brigade quickly won a reputation for being one of the toughest units of the Legion.
After heavy casualties in action and from disease the Legion was disbanded in December 1837. A quarter of the force (some 2,500 men)died,only half of them in combat.