From Wikipedia — origin of the word “Gringo” (this is the version I always heard of growing up in San Juan, Puerto Rico):
“When the Mexican-American War began in 1846, several hundred recently immigrated Irish, German, and other Roman Catholic Americans who were sent by the U.S. government to fight against Mexico came to question why they were fighting against a Catholic country for a Protestant one, combined with resentment over their treatment by their Anglo-Protestant officers, and deserted to join forces with Mexico. Led by Captain Jon Riley of County Galway, they called themselves St. Patrick’s Battalion (in Spanish, Batallón de San Patricio)[17] and frequently sang the song “Green Grow the Rushes, O”.[citation needed].
The 3rd Cavalry were the only U.S. Cavalry unit to wear green stripes on their trousers, and some believed that during their campaigns in the Southwest they were referred to as Gringos because of that stripe. Because of the prominence of Irish Americans in the regiment, the regimental song was “Green Goes the Rushes, Ho”. It is possible since the soldiers would sing this song as they rode on their horses, the Mexicans associated them with “Green Go...””
Ahhh, very interesting and informative...thank you.