Increased immigration from Ireland to the United States during the 1840s was primarily a result of

Potato famine / great famine

"The Great Famine (Irish: an Gorta Mór, [anˠ ˈgɔɾˠt̪ˠa mˠoːɾˠ]), or the Great Hunger, was a period in Ireland between 1845 and 1849 of mass starvation, disease, and emigration.[1] With the most severely affected areas in the west and south of Ireland, where the Irish language was primarily spoken, the period was contemporaneously known in Irish as An Drochshaol,[2] loosely translated as the "hard times" (or literally, "The Bad Life"). The worst year of the period, that of "Black 47", is known in Irish as Bliain an Drochshaoil.[3][4] During the famine, about one million people died and a million more emigrated from Ireland,[5] causing the island's population to fall by between 20% and 25%."

crop failures in Ireland that led to mass starvation