And you have to understand the attitudes at the time. My grandfather, a first generation American of Irish descent (both of his parents were Irish born) didn't have any negative attitudes about the English until he was forced to live there for months before D-Day. He spoke with a clear Irish accent, and was pretty shocked when he was subjected to serious British racism. He'd go to pubs with other American soldiers and be kicked out because they considered him an Irishman. He'd be made the butt of racist jokes. Some English people would refuse to discuss the war with him in the room because they didn't trust the Irish. Other than the lynchings, he said that the Irish were treated nearly as badly in Britain in the 1940's as blacks were in the American south.
There was a real hatred between the Irish and English people back then, and even the threat of Hitler wasn't enough to get the Irish involved to help the English (the fact that the Nazi's weren't really a threat to Ireland didn't help that argument). In that context, it's understandable why the Irish reacted so poorly when so many of them ran off to help the English.
FWIW, my grandfathers experiences in Britain in WW2 was also the foundation of his support for the IRA, which lasted right up until his death in the 1990's. He came home hating the English nearly as much as he hated the Nazi's (or, as he once said, "It's a damned shame that America went to war to save the Jews and gypsies, but left the Irish to whither and die" . Hatred breeds hatred, and his experiences were an example of that.