What caused the majority of Ireland's population to flee to other countries?

The great famine

Explanation:

The Great Famine was a time of starvation, sickness and huge displacements in Ireland somewhere in the range of 1845 and 1849. During the starvation, around one million individuals kicked the bucket and a million more emigrated from Ireland, causing the number of inhabitants in the island to fall somewhere in the range of 20% and 25%.

Famine is what caused it