The Irish Potato Famine, or the Great Hunger (An Gorta Mór in Irish), was a period of mass starvation, disease, and emigration in Ireland that lasted from 1845 to approximately 1852. The immediate cause was the widespread failure of the potato crop due to an airborne water mold, Phytophthora infestans, which causes a disease known as late blight. This pathogen, which arrived from North America and thrived in the damp Irish weather, destroyed the potato plants and caused the tubers to rot, often after they had already been harvested. Because a large portion of the Irish population, especially the rural poor and tenant farmers, relied almost exclusively on the potato for sustenance, the successive crop failures created a catastrophic food shortage. 

The impact of the blight was devastatingly magnified by existing socio-economic and political conditions under British rule. A system of absentee landlords and insecure land tenure meant that most Irish peasants were extremely poor and vulnerable, often farming small plots of land while other crops (like grain, meat, and dairy) were grown for export to Great Britain. British government policy, largely driven by a belief in laissez-faire economics, was inadequate and slow to provide relief, further exacerbating the crisis. While some initial aid was provided, such as public works programs and soup kitchens, the help was often insufficient and eventually hampered by bureaucracy and a banking crisis in Britain. 

The consequences of the Great Hunger were profound and lasting. An estimated one million people died from starvation and famine-related diseases like typhus and dysentery. Another one to two million people emigrated to North America and Great Britain, creating a massive Irish diaspora that permanently altered the demographic landscape of Ireland. The population of Ireland fell by roughly 25% between 1841 and 1871 and continued to decline for decades after the famine ended in 1852. The famine left a bitter legacy, fueling anti-British sentiment and intensifying Irish nationalist movements for independence.