Incidentally, a couple weeks ago, I had an Irish-American customer tell me about a part of southwestern history I'd never heard before — the San Patricios.
In the mid-1800s, many of the new Irish immigrants joined the military because it was one of the few employers who would hire them. However, just because they were hired didn't mean their officers had to like them. Most of the officers in the military at that time were protestants of English decent, and they didn't have much use for these dirty Irish with their drinking and papal idolatry. According to my customer, the Irish weren't allowed to go to Mass, and were often beaten by their commanding officers or thrown in the brig for minor offenses.
(He's Irish, so you have to take the story with a grain of salt. However, none of what he told me surprised me, or seemed out of step with what I know of Irish-American history.)
What is known for sure, is that many of these Irish soldiers found themselves fighting against Spain in the Mexican-American War, and large numbers of them deserted to fight on the side of the Catholic Spaniards. These Irish battalions came to be known as the San Patricios, and marched under a green flag. They were noted for their fierceness in battle, partly because the Irish have always been known for fierceness in battle, and partly because they knew they'd be hung as deserters if caught by the United States military. At one memorable -- and well-documented -- battle, a U.S. general ordered thirty San Patricios to be hung in full view of both armies at the pivotal moment in the fight.
Those San Patricios who survived the war were given land grants by the Spanish government in Mexico. Today, there are blue-eyed Mexicans with names like O'Connor and Mulligan who trace their heritage to the San Patricios who fought under a green flag in the Mexican-American war.