After defeating James II in 1690, victorious protestants subjected Catholics – Ireland’s majority population – to cruel restrictions on land ownership and leasing. These policies led most of Ireland’s people to farm plots that were inefficiently small and on which the Irish had no incentives to make long-term improvements.
As a result, agricultural productivity in Ireland stagnated, and the high-yield, highly nutritious, labor-intensive potato became the dominant crop.
In combination with other discriminatory measures that obstructed Catholics from participating in modern commerce – measures that kept far too large a portion of Ireland’s population practicing subsistence agriculture well into the 19th century – this over-dependence on the potato spelled doom when in 1845 that crop became infected with the fungus Phytophthora infestans.
There’s more more more here from the excellent Donald J. Boudreaux at George Mason U.