<<I don't know why they hated Irish back in the 19th century.
Aren't they one of the mainstream ethnicities in the US right now? >>
Right now, yes. Back then, no -- the Irish suddenly arrived in large waves in the 19th C following major famine and economic crisis in their country, and they were disliked back then for being Catholic, being poor, and being in large numbers -- pretty much the reason why most ethnic groups get disliked!
Aren't they one of the mainstream ethnicities in the US right now? >>
Right now, yes. Back then, no -- the Irish suddenly arrived in large waves in the 19th C following major famine and economic crisis in their country, and they were disliked back then for being Catholic, being poor, and being in large numbers -- pretty much the reason why most ethnic groups get disliked!