Yes! Irish people did survive on it for years - potatoes and buttermilk was what most poor Irish people ate in the 19th century before the famine. The potato blight hit so hard because they didn’t have a lot of alternatives, as the other things that were being grown there were were being shipped to Britain for the British landholders. (Coastal areas used to have fish but many people sold their fishing equipment to buy food the first bad year, in the expectation that the next year would be better and they could buy it back, but the next year was worse.)

They ate a really high volume of potatoes, however.