The story of the San Patricio Battalion is one of my favorite moments from US history, though perhaps the US doesn't deserve to claim the story since most of its heroes were recent immigrants from Ireland.
Anyway, if you're not familiar with the story, here's a brief summary via the website Latino Rebels...
St. Patrick’s Day is very special in Mexico because it is a time when Mexicans remember the San Patricios, or the Battalion >of St. Patrick. One of the least-known stories of the Irish who came to America in the 1840s is that of this Irish battalion >that fought on the Mexican side in the U.S.-Mexico War of 1846-1848. They came to Mexico and died, some gloriously in >combat, others ignominiously on the gallows. United under a green banner, they participated in all the major battles of >the war and were cited for bravery by General López de Santa Anna, Mexico’s commander-in-chief and president.
You can read the full story from Latino Rebels here.
Image courtesy of Wikipedia.