I'd like to know about the Irish great famine in the 1840s. Also, I want to know about the causes and symptoms of potato blight.

The great famine of Ireland took place in 1845-1849, and it is said that about a million people died of hunger during those years. In the early 1840's around a third of Irish people depended on the potato as their main source of food, and thus the destruction of the potato harvest because of potato blight in 1845 launched a catastrophe.

More information about the famine:
http://www.wesleyjohnston.com/users/ireland/past/famine/index.htm
http://www.bbc.co.uk/history/british/victorians/famine_01.shtml

Potato blight is a potato disease caused by a fungus. Its origins are in America, from where it spread to Europe by a shipment of seed potatoes destined for Belgian farmers in 1845. Potato blight first affects the leaves and later the actual potatos causing the to change colour and, eventually, turn into a soggy, foul smelling mass.

More information about potato blight:
http://www.allotment.org.uk/vegetable/potato/potato-blight.php
http://en.wikipedia.org/wiki/Phytophthora_infestans