The Great Famine (Irish: an Gorta Mór [ənˠ ˈɡɔɾˠt̪ˠə ˈmˠoːɾˠ]), also known within Ireland as the Great Hunger or simply the Famine and outside Ireland as the Irish Potato Famine, was a period of starvation and disease in Ireland from 1845 to 1852 that constituted a historical social crisis which subsequently had a major impact on Irish society and history as a whole. With the most severely affected areas in the west and south of Ireland, where the Irish language was dominant, the period was contemporaneously known in Irish as an Drochshaol, literally translated as "the bad life" (and loosely translated as "the hard times"). The worst year of the period was 1847, which became known as "Black '47". During the Great Hunger, roughly 1 million people died and more than 1 million fled the country, causing the country's population to fall by 20–25% (in some towns falling as much as 67%) between 1841 and 1871. Between 1845 and 1855, at least 2.1 million people left Ireland, primarily on packet ships but also on steamboats and barques—one of the greatest exoduses from a single island in history.

The proximate cause of the famine was a potato blight that infected potato crops throughout Europe during the 1840s, causing an additional 100,000 deaths outside Ireland and influencing much of the unrest in the widespread European Revolutions of 1848. Longer-term causes include the system of absentee landlordism and single-crop dependence. Initial limited but constructive government actions to alleviate famine distress were ended by a new Whig administration in London, which pursued a laissez-faire economic doctrine, and only resumed later. The refusal of London to bar export of food from Ireland during the famine was an immediate and continuing source of controversy, contributing to anti-British sentiment and the campaign for independence. Additionally, the famine indirectly resulted in tens of thousands of households being evicted, exacerbated by a provision forbidding access to workhouse aid while in possession of more than 1/4 acre of land.

The famine was a defining moment in the history of Ireland, which was part of the United Kingdom of Great Britain and Ireland from 1801 to 1922. The famine and its effects permanently changed the island's demographic, political, and cultural landscape, producing an estimated 2 million refugees and spurring a century-long population decline. For both the native Irish and those in the resulting diaspora, the famine entered folk memory. The strained relations between many Irish and their ruling British government worsened further because of the famine, heightening ethnic and sectarian tensions and boosting nationalism and republicanism both in Ireland and among Irish emigrants around the world. English documentary maker John Percival said that the famine "became part of the long story of betrayal and exploitation which led to the growing movement in Ireland for independence." Scholar Kirby Miller makes the same point. Debate exists regarding nomenclature for the event, whether to use the term "Famine", "Potato Famine" or "Great Hunger", the last of which some believe most accurately captures the complicated history of the period.

The potato blight returned to Europe in 1879 but, by this time, the Land War (one of the largest agrarian movements to take place in 19th-century Europe) had begun in Ireland. The movement, organized by the Land League, continued the political campaign for the Three Fs which was issued in 1850 by the Tenant Right League during the Great Famine. When the potato blight returned to Ireland in the 1879 famine, the League boycotted "notorious landlords" and its members physically blocked the evictions of farmers; the consequent reduction in homelessness and house demolition resulted in a drastic reduction in the number of deaths.