Why had potatoes become the main food of many poor people in Ireland before the famine?

Before the Great Famine, the potato became the primary food source for Ireland's rural poor because it was a hardy, nutritious, and calorie-dense crop that was relatively easy to cultivate in Irish soil.12

Many tenant farmers and landless laborers struggled to support themselves while simultaneously producing cereal crops for the British market. Given the small size of their land allotments, the potato offered an efficient way to sustain a family; a typical laborer's family could consume enough potatoes to provide approximately 80 percent or more of their daily caloric intake. By the early 1840s, this reliance had become so extreme that nearly half of the Irish population depended almost exclusively on the potato for their diet. This heavy dependence on a limited number of high-yielding varieties, however, created a dangerous lack of genetic diversity, leaving the entire population vulnerable when the late blight disease arrived.312