The main reason for the huge wave of Irish immigration to the United States in the 1840s was the Irish potato famine.  Beginning in 1845, a disease starting destroying Irish potato crops.  About half of Ireland's population at that time was made up of rural poor people.  There was essentially no social welfare system.  As the famine continued (it continued through 1849 and killed more than 1 million people), many Irish left Ireland to come to America.  It is estimated that between 1 and 1.5 million left Ireland, going to the US, Canada, and Australia.  They hoped to have a better chance at survival and even prosperity there.