The San Patricios have a somewhat cloudy early history, and very little is known for certain regarding their recruitment. One popular tale (played upon in the film One Man's Hero) says that the nucleus of the unit was formed following the severe punishment of Roman Catholic soldiers who had attempted to sneak into Mexican territory and attend mass. While this probably played a role, it is known that a large number of Irish, Czech, Hungarian, and German immigrants deserted the U.S. Army shortly before, and during the prosecution of, the Mexican-American War. They seem to have been motivated by the widespread discrimination practised within the U.S. Army at the time against Roman Catholics and a number of well-documented instances of rape and pillaging by American soldiers in largely Catholic Mexican border settlements.
The San Patricios first emerged during the Battle of Monterrey, as a battery of artillery commanded by a former United States Lieutenant and Irish-born immigrant, Jon Riley. Here, they served with great distinction, and are sometimes credited with defeating two separate American assaults into the heart of the city.
Following the engagement at Monterrey, the San Patricios grew in number and probably numbered around at least eight hundred men. Despite their excellent performance in a number of engagements as artillery, the San Patricios were ordered to muster as an infantry battalion in mid-1847 by personal order of Antonio López de Santa Anna. At the Battle of Churubusco, they were nearly annihilated, with most either being killed, or being taken prisoner (including Jon Riley). They were briefly reformed just before the Battle of Mexico City, but never regained their former numbers and were mustered officially out of Mexican service in 1850.
The surviving San Patricios taken by the U.S. Army suffered harsh reprisals. Those who had entered the service before the official declaration of war (Riley among them) were branded with the letter "D" as deserters and sentenced to time in hard labor before being released. Those who had entered the service following the declaration of war were hanged en masse in full view of the Battle of Chapultepec. As per the orders of General Winfield Scott, at the precise moment that the flag of the United States replaced that of Mexico atop the citadel they were executed.
Those who were released and survived the war generally disappeared from history. A handful are on Mexican record as having made use of land claims promised them by the Mexican government.