Between 1845 and 1850, the Great Irish Famine was caused mainly by a lack of potatoes, but also by blights on potatoes, thus the alternative name, "The Great Irish Potato Famine".

One of the most tragic events in Irish history, the Great Irish Potato Famine, claimed the lives of at least one million, as well as being the main reason for mass-emigration from Ireland to other countries like the USA, the UK, Australia or Canada. This caused Ireland's population to drop by as much as 25%. The famine had its largest impact in the Irish-speaking counties (Irish states) which were present in the South and the West. The Great Irish Famine wasn't only one of the largest famines in World history, but also one of the largest migration periods from a single island in World history.

It was a time of utter grief and starvation, where everybody was fighting each other for the little food that still existed in Ireland back then.

More Info: en.wikipedia.org