Between 1845-1850, a potato blight water mold struck across Europe. It turned this important food staple into a black, soggy, and inedible mess. In Ireland, the lower classes were particularly dependent on the potato as their primary and sometimes only foodsource. It is estimated that the initial population of Ireland, about 8 million, was reduced by a quarter, with 1 million dying and 1 million emigrating, primarily to America. During this time, Great Britain forced the Irish to export corn (and other crops) which could have saved the lives of many Irish.
Potato blights continued in Ireland, especially in 1872 and 1879-1880. These killed few people, partly because they were less severe and partly because Irish-Americans contributed to relief efforts. They did lead to reform in the British agricultural and land-owning laws and to continued emigration. By the 1890s, the Irish population had fallen to around 4 million, about the same as the population in 1800 and 2000 and only a half of its peak population. The same mold (Phytopthera infestans) was responsible. When people speak of "The Irish potato famine", they nearly always mean the one of the 1840s; there is much less awareness of the later ones.