Good heavens I had no idea there were so many songs about this.
I'm going to quote liner notes off the "Soldiers' Songs" CD by Captain Mackey's Goatskin and String Band (Jimmy Crowley and Martin Cogain)

The Men that God Made Mad
Ron Kavana wished to record the heroism of the Battalon de San Patricio in the Mexican-American war (1846-1848) who defected to the Mexican side and were executed by the American Army. To the United States, the deserters are traitors; but to Mexicans, the "Irish martyrs" are heroes, honored in street names, plaques and St. Patrick's Day celebrations around the country. The battalion's name is written in gold letters in the chamber of Mexico's House of Representatives.

Far, far from Clifden's rocky shore o'er the broad Atlantic sea,
The battalion of St. Patrick tired of harsh brutality
No more abuse or bigotry, their angry cry whole-hearted
Near Matamoros lives were lost, that's when the fighting started

Chorus
Who were those men, what was the crime
For which their lives were wasted,
Did they rob or rape or was their fate
As the poet once related?
Were those great Gaels of Ireland
The men that God made mad?
Their wars were never merry,
But all their songs were sad

Land of the Free meant liberty to the U.S. Army's Irish,
Til James K. Polk he sent 'em south to civilize the Spanish
In a war to extend slavery and unjust exploitation
They'd not repeat what Cromwell did to their own poor Irish nation.

At Angostura Irish blood drenched the sun-baked clay
And Mexico still honours those brave men who died that day
But the worse was yet to come in the hour that was ended
When General Scott hung Irishmen to celebrate with vengeance.