I have heard - don't ask me for citations, though - that the 'Scotch-Irish' are now the poorest ethnic group in America.

Seems to me that you can most helpfully teach this by looking at how people who might be a powerful force if they worked together - for example, the working class - are neutered if they're divided by what the Irish Declaration of Independence calls "differences carefully fostered by an alien government".

You might show your students the film Ragtime - not to make them hate, but to show them how one working-class group can turn against another that threatens its tiny margin of advantage, if that advantage is so endangered that its owners are paranoid.