The catastrophic failure of the Irish potato crop from 1845 to 1852, which killed approximately one million people and drove another million to emigrate within a year — reducing Ireland's population by a quarter and setting in motion a century of emigration that permanently transformed both Ireland and the Irish diaspora.
A million people died. Another million fled in a single year. The British government shipped food out of Ireland while people starved. Whether this was genocide, criminal negligence, or ideological failure is still debated. The Famine is not history in Ireland — it is the wound through which everything else is understood.