Ireland
The Potato Famine
After the famine the population of Ireland declined further as emigration continued and the birthrate fell. By the 1911 census, the island’s population had fallen to 4.4 million. Although it has since increased, even today the total population of the island of Ireland is only around 6.4 million, which still leaves it 1.8 million (or almost 22%) less than it was in 1841.
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30% of Irish people were largely or wholly dependent on potatoes for their food. To make matters worse, they were largely reliant on a single variety, the Irish Lumper, so that the entire crop failed at the same time due to lack of genetic variation.
See also the Wikipedia article on the European famine generally, The European Potato Failure. Potatoes were 32% of the arable land in Ireland, 11% in Prussia.
