If one reads the writings of Southern politicians and leaders before the Civil War, you will notice that many, if not the vast majority, of Southern leaders wer strongly pro-Irish Catholic and strongly against the Know-Nothings. I could go and dig up some interesting quotes from several pre-War governors of Southern states that express views that would shock most in their seeming "tolerance", tolerance of other ethnicities and religions not being sometihng we have been led to associate with the Confederacy. While anti-Irish sentiment was very strong in the North, it was all but negligible in the South, at least in political action. Of ocurse, there was some debasing of Irishmen in the South, but overall it was a fine picture for sons of Erin in the South. The Irish Catholic Church in the South was also strongly on the side of the Confederacy, including one priest who the Confederacy dispatched to the Pope to appeal for an end to Union recruitment in Ireland. Evidently it worked to a great degree, though it was very late in the war and thus had little effect.
Incidentally, Mr. Patrick Cleburne, an Irishman who settled in Arkansas, and later became a Confederate general, had originaly been a Whig, but converted to the Democrat party (the prevelant party of Southerners and Irishmen of the day) after a Know-Nothing shot him in the back while Cleburne was walking down the street. Let's be glad political affiliation rarely leads to such things today!