In 1800, some five million people lived in Ireland. By the autumn of 1845, when the Great Famine struck Ireland, there were more than eight million. Many of them were wretchedly poor, eking out a precarious living on tiny plots of land, and dependent on each year's potato crop. Hunger was no novelty to peasant families, for there had been partial failures of the potato crop in other years. However, these had always been of limited duration, and confined to a small number of counties. The Great Famine lasted from 1845 to 1848, and crop failure affected the whole island.

The cause of the famine was a fungus disease which caused the potato plants to rot in the ground, giving off an appalling stench. The blight first destroyed crops on the eastern seaboard of America in 1842, then appeared in England in the summer of 1845. In September, the counties of Wexford and Waterford reported the disease. More than half the Irish potato crop failed in 1845. Sir Robert Peel, the British prime minister, appointed a commission to investigate the problem, but scientists were unable to explain the disease, let alone find a cure. In 1846, the potato crop was a total failure.

Peel, to his credit, also introduced relief measures.   In November 1845, the government spent £100,000 on buying grain from America, in the hope of keeping food prices down in Ireland. He appointed a relief commission which set about forming local committees to raise money and to distribute food. At Westminster, in part prompted by Ireland's problems, Peel succeeded in repealing the protectionist corn laws in June 1846. This opened up the prospect of cheap imports from America. A month later he was out of office, defeated over a bill to deal with the growing agrarian disturbances in Ireland.

The new Whig government, led by Lord John Russell, believed in a free market and was content to leave the supply of food to private merchants. However, the Irish peasants were unused to a cash economy, for they had traditionally worked for a landlord in return for a plot of land on which to grow potatoes. The government hoped that Irish landlords would bear the major responsibility for their tenants' welfare, but many landlords already faced ruin. The most successful relief came from soup kitchens, originally set up by bodies such as the Society of Friends. Where public works continued, they were often delayed by bureaucratic procedures, and workers' health suffered from the inadequacy of wages to buy what food was available. Evictions were common.

Even the weather contributed to the distress, for the winter of 1846-47 was exceptionally cold and wet. To starvation was added typhus and relapsing fever, both commonly called "famine fever". Scurvy and dysentery flourished, and in 1849 an outbreak of cholera claimed many lives, particularly in the larger towns. Many sought to escape to America, only to drown at sea in over-crowded "coffin ships". Those who did reach the New World were often weakened beyond recovery.

Eventually the government reformed the poor law system, so that outdoor relief was added to the limited accommodation of the workhouses. Medical services were improved with the establishment of temporary fever hospitals. By the end of 1849, the potato blight had passed and crops returned to normal. About one million people had died, and another million had emigrated. The population continued to decline, not only through emigration but through later marriages, lower birth rates and an end of the sub-division of farms which had made Ireland so vulnerable to crop failure. The famine was to prove a watershed in Anglo-Irish relations, for the inadequacy of government measures left an enduring legacy of bitterness in Ireland and among those thousands of Irish emigrants who found a new life across the Atlantic.