Wikipedia says on the origin of the song:

A ballad by the name "Spanish Ladies" was registered in the English Stationer's Company on December 14, 1624. The oldest mention of the present song does not, however, appear until the 1796 logbook of HMS Nellie, making it more likely an invention of the Napoleonic era. The timing of the mention in the Nellie's logbook suggests that the song was created during the War of the First Coalition (1793–96), when the Royal Navy carried supplies to Spain to aid its resistance to revolutionary France. It probably gained in popularity during the later Peninsular War when British soldiers were transported throughout the Iberian peninsula to assist rebels fighting against the French occupation. After their victory over the Grande Armée, these soldiers were returned to Britain but forbidden to bring their Spanish wives, lovers, and children with them.