Xenia, it wasn't all the potato.
The population was allowed by landlords to subdivide tiny farms into mere fields and potato patches, each generating rent. The potato could be grown reliably, and was eaten in great quantities along with butter and buttermilk, the occasional meal of pork/boiled bacon/pigs' feet/other pig parts, organ meats, and maybe some chicken, certainly eggs. Pigs were kept and sold to pay the rent, and beef cows too. Maybe families had some mutton and lamb in their diet, though sheep destroyed mountain pastures. Along with that went oats - porridge, oat cakes, and other permutations and in some parts, fish and seaweed from the ocean. Fish from inland waters was only available if poached and penalties for this were harsh. In the poorer parts (Connemara, west Cork, Donegal and other places) there was less meat, though traditional transhumance practices were common with cows; oral records undertaken in the early years of Irish independence reveal that young people accompanied the cows up the slopes (and also had sleep patterns common many hundreds of years ago that have died out now - 'first sleep' to midnight, a few hours up, followed by 'second sleep').
The blight came and the potatoes rotted. The famine could have been contained and the effect of a sudden and drastic potato shortage mitigated if government policy had not welcomed the thinning out of the population as a good thing, and the freeing up of formerly occupied land for sheep grazing as 'progress'.
I read somewhere that the diet of your average Scottish Highlands and Islands resident was vastly superior to that of English farm labourers in late Victorian and Edwardian times. The reason was the availability of fish, mutton, lamb, (including fish oils, lard, and animal livers), a little game, chicken, eggs, oats and root and winter vegetables and the brassica family in Scotland. Crofters grew their own food and foraged for blueberries and blackberries, etc.
www.westonaprice.org/health-topics/traditional-diets/the-good-scots-diet/