Maybe because the Irish came over in huge numbers and are responsible for helping build up America's infrastructure and manufacturing more than anyone else.
Could also be that the Irish were persecuted against pretty heavily here in America and in Ireland. Not so much the Scots and Britts
Yeah, like Italians, the Irish weren't considered White. Whiteness was a concept invented to justify slavery and human exploitation in the New World. It's interesting to see how different races have become accepted as 'White' over time.
A book I highly recommend is White Fragility by Diangelo.