In 1845, a blight ruined Ireland’s potato crop. A single-crop failure should not have caused a deadly famine. But a national disaster quickly followed when other crops - which had NOT failed - were exported.
With nothing to sell, potato farmers had no money to pay rent. Landlords evicted families from their homes as starving mothers begged for food. Fathers, trying to provide some type of shelter for their families, carved "hovels" out of Irish bog. Death had descended on the Emerald Isle.
Those who survived had one thought: leave Ireland. Within four years, a million people fled the country. But "Famine Ships" became "Coffin Ships" as already ill people could not survive the crossing to America. By the time ships arrived in Boston or New York, they were far less crowded than when they had left Europe.
Once in New York, immigrants from the same country tended to live in the same neighborhoods. And young men from those neighborhoods tended to form gangs. Among the most notorious gangs, in the mid 1800s, were those from the Irish section of Five Points.