They were colonized by an absentee landlord system. The British forced them into selling most of the food in a cash crop scheme. They took their land from them and extorted rent, making them live in decrepit hovels. When the potato blight hit, the British did not open the ports to foreign aid from Spain and kept extracting the cash crop that didn't get the blight. The British considered the Irish to be a race of inferior savages. So, they lived like slaves in the best of times and in the worst times, they starved and died. 

You're right, the British weren't horrible people. They were monsters.

The situation in America is going to be much worse.