Some time during this Saint Patrick's Day weekend, if you can find it, you might want to watch One Man's Hero, starring Tom Berenger. It's a film about the hundreds of Irish Americans who left the U.S. Army to fight for Mexico at the beginning of the Mexican-American War, 1846-48.
While the reasons for Irish-Americans to leave the U.S. Army are many - brutal treatment of the Irish at the hands of U.S. military officers, economic incentives from Mexico, etc. - the broadly accepted rationale is that many Irish felt strongly about fighting against other Catholics (though, there are also stories of many Irish-American U.S. soldiers attending Catholic mass in Mexico when war was declared, which made them automatic deserters in the eyes of their commanding officers).
Whatever the reason for deciding to fight for Mexico, the San Patricio battalion story is a fascinating one, and now serves as a point of mutual admiration and respect between Mexico and Ireland. Mexico even has plaques and memorials honoring the San Patricios who fought alongside Mexicans as the nation fought against the "unjust North American invasion of 1847."
Below are a couple of links that help explain the San Patricio story, and one that explains the historical and cultural affinity between Mexicans and the Irish in the United States.
On Saint Patrick's Day, Mexico remembers the Irishmen who fought with them against the U.S. (PRI).
The soldiers of Saint Patrick (Latino Rebels).
The Irish were the original bad hombres (Chicago Tribune).
Then we have this 2017 clip, courtesy of Ireland's Prime Minister, Enda Kenny.