The primary reason that large numbers of people left Ireland in the 1840s and 1850s was MASS STARVATION.
Great Famine or Great Hunger was a period in Ireland where mass starvation, diseases, and emigration occurred between 1845 to 1852.
The Great Famine was caused by Phytophthora infestans, a disease that infests in potatoes. Though the potato infestation spread throughout Europe, it greatly affected Ireland, where 1/3 of its population was dependent on the potato crop as their primary source of food.
The loss of their primary source of food led to mass starvation, sickness, and eventual death. Others opted to leave Ireland and seek shelter and food in other countries to survive.