Irish Potato Famine: Why did so many people starve to death during the Irish potato famine?

From my knowledge, potatoes used to be a poisonous plant. Unbelievable right? So what the farmers whould do is they would stomp on them and let them be in the sun for a while then the heat of the sun would rise the poison so the potatoes wouldn't be poisonus anymore so then they are safe to eat.