What we refer to as Scotch-Irish that had settled in the Appalachians are the descendants of indentured servants/penal colonists. They were brought here against their will. They were an unwanted criminal class that England wanted to get rid of, and there was a labor shortage here in the Colonies, so that was two birds with one stone.
Irish started immigrating en masse mid-19th century on account of the Famine. Many ended up in the Five Points neighborhood (as featured in Gangs of New York) in Manhattan. It was so bad, police would not enter. The gangs made an arrangement that they would dump all dead bodies on the edge of the neighborhood for the police to pick up. Lore has it that there was a murder at night, although that was probably a bit of an exaggeration. Since the police wouldn't enter the district, they were never investigated.
The neighborhood was so bad, they're the reason why police first in Manhattan and then the rest of the United States of America carry guns to this very day.
As the years went on, the Irish started to assemble, protest, and riot in response to discrimination, exploitIon, and being paid substandard wages. Thanks to them busting heads like no group before or after, we have labor unions and various rights at the workplace that we didn't have prior and that they fought for.