St. Patrick's Day is a Catholic celebration. Both countries have a Catholic majority among their respective populations.

Also, there was a military unit, "The St. Patrick's Battalion" which fought during the Mexican-American War (1846-1848). As many of the combatants of such group were Irish immigrants into the United States, they were mistreated by their American commanders, thus prompting them to switch sides and fight for the Mexicans. Although they are considered traitors by the United States, the San Patricios are regarded by Mexico as heroes who came to the aid of fellow Catholics in need.