Original sentence: When the Scots-Irish first arrived, they were perceived as a distinctive group who settled mostly in the backcountry; not only were the Irish Catholics a much larger group arriving in a later era of immigration, but they were at first separated from the main society by their Catholic religion and also by the long tradition of oppression by the English
Edited sentence: When the Scots-Irish first arrived, they were perceived as a distinctive group who settled mostly in the backcountry; not only were the Irish Catholics a much larger group arriving in a later era of immigration, but they were at first separated from the main society by their Catholic religion and also by the long history of oppression by the English