Ireland's great famine of the mid-19th century resulted in:

Millions of people fleeing Ireland for the Americas

Explanation:

The great famine of Ireland was a period of illness, death and misery that occurred in the nineteenth century (1845-1852), which killed one million people and forced the emigration of millions of other Irish to North America. According to historians, the great hunger occurred because something like a fungus contaminated a large volume of potatoes, making it impossible to consume them and starving the Irish.

Great Famine, also called Irish Potato Famine, Great Irish Famine, or Famine of 1845-49, famine that occurred in Ireland in 1845-49 when the potato crop failed in successive years. The crop failures were caused by late blight, a disease that destroys both the leaves and the edible roots, or tubers, of the potato plant.