There is not a ton of info on foreign units in the Spanish Army during the Napoleonic Wars, either being overshadowed by France's extensive use of foreign troops or Spain's lack of foreign troops. I could only really find information on Swiss units. One particular unit which has a decent amount of information, in English at least, is the
3rd Swiss Regiment Reding which was part of the Spanish Army before and after the French invasion. There is not much information on the regiment during the initial French invasion other than that they were part of the Pyrenees front during the War of the First Coalition. The regiment seemingly did not disband after the French victory and joined the Spanish forces against France during the Peninsular War. Specifically, the regiment allegedly served with distinction at the Battle of Bailen, which was one of France's largest defeats of the Napoleonic Wars. I don't know what the regiment did about casualties as there is so little information but my guess is they couldn't or didn't replenish troops. The 3rd Swiss only fought in two large battles, and only seemed to play a significant role in one of them.
The Napoleonic Wars is a couple decades after Spain was a large continental power so it is very unlikely that any Italian or Walloon troops fought in the Spanish Army as separate units. I may be wrong but I simply can't find any information on this. The same is similar for Irish units, while it is entirely possible that they could have existed, it seems unlikely as there is no information on them for this particular era. There is ample information on troops of these nationalities in service of France and they seemed to perform just as well as any French unit.
