I’m referring to the usual mainstream American accent that Americans speak with. Where did it come from?
I'm English and I occasionally mistake some Irish accents for American until I listen very carefully
According to the Dictionary of American History, approximately "50,000 to 100,000 Irishmen, over 75 percent of them Catholic, came to United States in the 1600s, while 100,000 more Irish Catholics arrived in the 1700s." Indentured servitude was an especially common way of affording migration, and in the 1740s the Irish made up nine out of ten indentured servants in some colonial regions.