In the year 1845 a blight destroyed the annual crop of potato, which is what the vast majority of Irish people lived on. For four years blight would ravage the potato crops and would contribute to what some have argued might have been between a million to a million and a half deaths. What further contributed to the misery was the lack of assistance from the British rule.

Source:http://www.victorianweb.org/history/famine.html