Most immigrants came from Ireland due to the potato famine.
Step-by-step explanation:
One of the most devastating things to happen to Ireland was the potato famine where millions of people died of starvation in a famine tat was largely avoidable but the British did not do anything about it.
A huge stretch of the population thus decided to move away from the nation to settle for better prospects and the majority chose US.
The US thus became swarmed with migrants from Ireland on its shore that wanted sanctuary.