I wrote about the San Patricios, an Irish regiment of the Mexican army, in Fadó. Its members were previously in the US army, but deserted after enduring discrimination.
Their story is told here in the film "One Man's Hero" with Tom Berenger in the lead role. Given that it is not quite the flag waver movies the Americans like, it did not get the recognition it deserves.
My thanks to Bob Angelo for providing the link.