Because of the Brits (boo, hiss) poor Irish farmers who lived off the land no longer had large enough farms to grow enough vegetables to live off . The potato is a relatively high-yield crop that produced enough food (just) to keep them alive, so all they had to eat was potatoes, plus a *very* small amount of meat and other veg if they were lucky, and small amounts of dairy products. The blight wiped out the potato crop, leaving practically nothing for them to eat - similar to when the rice crop fails in parts of Africa. The population of Ireland dropped from 8million to about 1 million during the famine , and still hasn't recovered. Today's population is about 4 million.