It was caused by the same type of floods that the UK has experienced in the last year (yep, they don't tell you but it has all happened more than once and is not down to Tesco plastic bags and carbon footprints).

Nothing, honest, It wasn't me...

The blight wiped out all or most of the potatoes grown in Ireland at that time:-)

The failure of the potato crop was caused by potato blight – the parasitic fungus phytophthora infestans. The blight spread to Ireland from Britain and the European continent, its spores being carried by insects, wind, and rain. Symptoms of infestation in the plant included black spots and a white mould on the leaves; the potatoes would rot rapidly into a pulp. However, the underlying cause of the disaster was the weakened condition of Irish agriculture. The effects of the Navigation Acts, absentee English landlords and the extortions of their land agents (which included rack rents – excessive and frequently increased rents), absence of compensation for improvements, restricted rental holdings, the anti-Catholic measures of the penal code, and large families, with consequent subdivision of land through partible inheritance, had combined to create a system of impoverished, tiny Irish allotments, whose tenants had little opportunity to diversify and relied largely on potatoes for subsistence. Pigs, an important source of cheap meat and income for Irish farmers, were also fed potatoes as a basic fodder. When a potato blight destroyed the crop four years running from 1845, the result was famine. Cereal harvests remained excellent, but the prices were too high for the poor. Although in 1845 Robert Peel's Tory government reacted with a grant of £100,000 to purchase Indian corn from the USA, and the abolition of the Corn Laws in 1846, the Liberal government of John Russell placed responsibility for dealing with the disaster on the Irish landowners, under the terms of the 1838 Poor Law. Efforts to cope collapsed as the starving flocked to the local poor-law boards and as workhouses became overcrowded and rampant with disease. Private philanthropic organizations and, for the most part, all religious denominations, particularly the Society of Friends (the Quakers), worked to provide relief. However, deaths began to mount in late 1846, and the famine reached its peak in 1849. Massive emigration occurred, particularly to the USA.