Picked up a book about this unit & it's interesting.   There are a couple references to them in "Gone for Soldiers" by Jeff M. Shaara

Were they the Irish Catholic traitors who defected to the Mexicans in the Mexican-American war?

Supposedly the source of the word "gringo"?

Yep.  At the time, there were many Irish entering the U.S. & there was a Nativist movement in the U.S. which was basically anti-immigrant & anti-Catholic.  The 2 clashed.
Many of the newly arrived Irish (with previous Brit military service) who could not find jobs entered the military & there found instead of chances of promotion, abuse at the hands of Nativist officers.

The abuse played right into the appeals of the Mexicans asking the Catholics to come serve them.