Historically, most European immigrants pre-ACW, passed through Ellis Island on the way to New York City and American citizenship.
What's the strongest and earliest PoD that would instead cause New Orleans to be said path way? Eclipsing New York City as the through way for at least 80% of all immigrants
How would this effect Dixie culture pre-ACW?
Would there be a multicultural slaving holding class? (Irish, Jews, Germans, British, etc.)
How would this effect the ACW?
I'm afraid you might a fairly radical POD for this, which would have to involve significant changes to the local culture of not just N.O., but perhaps of Southeast Louisiana as a whole.....including less outright support for slavery, and more of merely tolerating it; one of the reasons why few Irish or Italian Americans came to this area before 1880 was due to the slavery problem-even many German Americans were anti-slavery(a good part of that, it seems, may have indeed been due to the liberal sentiments of many of those who had fled continental Europe following the failed revolutions of 1848); in fact, for the South as a whole, only Texas saw any significant numbers of German immigrants IOTL, and that was only thanks to the Adelsverein and other organizations who had encouraged settlement in that area during the Republican era.
As for a multicultural slave-holding class? I'm afraid that even in New Orleans, this would be rather difficult-a few Jews, Protestant Germans, and maybe even Ulster Irish might be allowed in, but it's rather likely that pretty much everyone else would be shut out, as anti-Catholic sentiment was also fairly strong down South.