Yes. Next question, please.
Some of the Irish in New York City were sympathetic to the Confederacy, as Ireland's desire to break away from the United Kingdom resembled Dixie's desire to secede from the United States. Also, Southerners saw themselves as the successors of the English Cavaliers, who were allied with the Irish Catholics on the Royalist side during the English Civil War. The Anglo-Protestants of the Northeast were, to a considerable extent, the descendants of Puritans who sided with the Roundheads, the Royalists' opponents. None of this was depicted. Indeed, the nativist leader played by Daniel Day Lewis was shown as anti-Lincoln and pro-slavery while DiCaprio's Irish gang boasted its very own "token Negro." In fact, most Know-Nothings became Republicans after their movement collapsed.
There are other examples of PC thought in this movie. "Faith based" social help is shown as hypocritical and obtuse, as reflected by DiCaprio's snotty attitude toward it (tossing a King James Bible he received at an orphanage into a river; telling a Protestant minister running a charity dance to "go to hell" when he mentioned when services were held). When Cameron Diaz engages in theft, or Leonard DiCaprio and his friend steal the prized possessions of a fellow Irish immigrant whose home caught fire, their poverty and oppression, of course, justify their thievery.
What production qualities were in the movie, as well as Daniel Day Lewis' performance, are outweighed by the miscasting of DiCaprio and Diaz, the historical inaccuracies, and the leftist ideology in the story.
I think you're the one propagating a myth. Smith swept the states of the Old Confederacy, where Catholics were about as plentiful as hen's teeth. Hoover kicked Smith's crooked butt in the rest of the nation (including the heavily catholic Great Lakes states and Northeast.