What was one of the main reasons that large numbers of Irish moved to the United States in the 18405?

Potato Blight

Explanation:

Ireland's 1845 Potato Blight is often credited with launching the second wave of Irish immigration to America. The fungus which decimated potato crops created a devastating famine. Starvation plagued Ireland and within five years, a million Irish were dead while half a million had arrived in America to start a new life. Living conditions in Ireland were deplorable long before the Potato Blight of 1845, however, and a large number of Irish left their homeland as early as the 1820s.