Why do some Irish people hate the English?

Don't think that it's hatred.

It seems to be resentment that's easy to create in people and forms half of AH threads.
Whether it's a Joe Duffy / Irish Independent complaint about the public sector, government, EU, ryanair ; travellers feeling disrespected by non-travellers and vice versa; or a biased retelling in a dodgy school history book.

Most of us don't hate politicians, but think that they have no respect for us when they speak to us as though we're simpletons in their efforts to 'build confidence'. So resentment is returned.

Equally there's a taught false perception that the english as a whole think of us with contempt; when you see that infamous Punch cartoon that keeps being dragged up almost 150 years later, someone's trying to manipulate you.
The indo tries to drum up the same every time some german politician says something foolish regarding Ireland.

In reality it's a rare enough oddball that tries to elevate themselves by disrespecting others, and there are plenty of the same that are Irish, including family members.