Two off the cuff thoughts about the Irish diet circa 1800
1. They were still rather fertile so their health couldn't have been too bad. This makes me think milk and potatoes must have had a lot more merit to it than we're thinking. Nowadays a lot more women struggle with infertility from nutrition issues, though perhaps is the packaged food or chemicals and pesticides.
2. I've heard that people of Irish descent nowadays actually struggle from having too much iron in their blood (the opposite of anemia) and have to donate blood regularly. This makes me inclined to think that people with Irish genetics may be more able to pull iron out of lower iron foods. Perhaps plant iron that is not bioavailable to many people is still bioavailable to people with Irish genetics. I have often wished for this gene, there's times in my life where I've been borderline anemic for years because steak was expensive. Don't worry, nowadays I spend a lot more on food and I'm no longer anemic.
3. If the FDA guide is worth anything (I used to treat it really seriously and now I'm not so sure, there are so many micronutrients and phytonutrients and bioavailability issues they don't account for), you could run through the official list of all the different vitamins and minerals they say are required and see which has which. I think at one point in my life I worked out an attempt at nutritional completeness for the cheapest possible price that came down to: sweet potatoes + white potatoes + chicken liver and onions + sunflower seeds. At the time I thought it was nutritionally complete, though right now I'm not poor enough to attempt the experiment again. If you do go this route, be careful not to overdose on chicken liver (it has exceeding amounts of the real type of bioavailable vitamin A which is really good for you except if you overdose and then it's poisonous) and keep sunflower seeds in the fridge because they go rancid lickety split once a bag is opened at room temperature. Also, I recommend against eating rancid sunflower seeds. I did it and it was a huge mistake, and I didn't get the nutrition as it causes bowel issues...
EDIT: do put fat on your potatoes. I just remembered that when I lived off of microwave potatoes (without fat to save money) for about a month, I became pre-diabetic. It causes glucose spikes and taxes your natural insulin system. The Irish would have avoided this problem with full fat milk and butter on their potatoes, which would have cut the glucose Spike and prevented diabetes. There's a reason everyone eats their potatoes with lots of fat.