Great Famine

The Great Famine was caused by a failure of the potato crop, which many people relied on for most of their nutrition. A disease called late blight destroyed the leaves and edible roots of the potato plants in successive years from 1845 to 1849.

As a direct consequence of the famine, Ireland's population fell from almost 8.4 million in 1844 to 6.6 million by 1851. About 1 million people died and perhaps 2 million more eventually emigrated from the country. Many who survived suffered from malnutrition. Additionally, because the financial burden for weathering the crisis was placed largely on Irish landowners, hundreds of thousands of tenant farmers and laborers unable to pay their rents were evicted by landlords unable to support them. Continuing emigration and low birth rates meant that by the 1920s Ireland's population was barely half of what it had been before the famine.

The potato plant was hardy, nutritious, calorie-dense, and easy to grow in Irish soil. By the time of the famine, nearly half of Ireland's population relied almost exclusively on potatoes for their diet, and the other half ate potatoes frequently.

The Irish relied on one or two types of potatoes, which meant that there wasn't much genetic variety in the plants (diversity is a factor that usually prevents an entire crop from being destroyed). In 1845 a strain of water mold accidentally arrived from North America and thrived in the unusually cool moist weather that year. It continued to destroy potato crops from 1846 to 1849.

About one million people died during the Great Famine from starvation or from typhus and other famine-related diseases. An estimated two million more emigrated from the country.