Another discussion about the humble "pratie" on this forum prompted me to look online to see if I could find an article discussing how well one might survive on what the Irish peasents diet was reported to be in the mid-19th century. This article does a pretty good job of analyzing the nutritional value of the "milk and potatoes" diet. It concludes that it's close to being enough, but you would need to suppliment it a little, proabaly with either lentils, split peas, green beans, cauliflower, or oatmeal, although you would not need too much of one of those per day.

Here's the article: Could I survive on nothing but potatoes and milk?