I read a bit from Black Rednecks and White Liberals which points out that perhaps the reason why anti-Irish sentiment went away was because the Irish changed.

I offer my hypothesis as to why the Polish were thought stupid.

Then I tell the story of a fascinating man, Garrett Morgan.  Morgan invented the first good gas mask which he used to save 29 men suffocating below Lake Erie in 1916.

Morgan was later the pioneer of traffic signals, embellished his own triumphs, discovered a hair-straightening solution and marketed skin-bleaching to blacks.  He was discriminated against, but in the end he was acknowledged a hero.