Italians, actually (FDR creating Columbus Day, even though Chris never set foot on soil which is now American).
Irish were split between support of Northern Republican elites and Tammany Hall.
The Democrats lost the South when LBJ signed the Civil Rights Bill.
Black voters moved from the Party of Lincoln during FDR's admins, since lifting the country out of the Depression also greatly improved the lives of Black Americans. They feared Truman would be a Southern Dem (he had some uncomfortable associations with KKK and was quite bigoted) but after meeting with Black Leaders, and assuming mantle of president, of all Americans, Truman not only promised to continue FDR reforms, but did, overcoming his own prejudices.
Meanwhile, Nixon's Southern Strategy seems to have had some legs, and even to this day, the GOP is on the wrong side of minority issues, reflexively ... and of course, running out of enough Angry Whites to offset the now unbeatable coalition of Liberal Whites and virtually every minority group, except anti-Castro Cubans, who are now even coming over to the Dem side due to the GOP being so anti-Hispanic in its rhetoric.