You got it! -- The Scots-Irish were/are descended from the (mostly Lowland) Scottish planters whom the English sent to Ireland during the Plantation Era, which saw the beginning of contemporary Ireland's colonisation. Plantation enjoyed the most success in the northernmost province of Ulster, which means that the Scots-Irish (as we might refer to them) were/are concentrated in the northeast parts of the island of Ireland. Many Scottish planters were Presbyterians, or "dissenters," and suffered certain legal, economic, and political restrictions under the Penal Laws in Ireland, which mostly targeted Catholics, but affected some other non-conforming (to the Church of Ireland) religious groups as well. Discrimination against Presbyterians meant that they were prominent in Irish reform movements throughout the 19th century, most particularly in the United Irishmen uprising in 1798. Those who remain in Northern Ireland today often refer to themselves as "Ulster-Scots," and see themselves as a distinct cultural group. I don't know too much about the Scots-Irish diaspora in America, unfortunately, but you'll definitely want to check out Warren Hofstra's (ed.) Ulster to America: The Scots-Irish Experience (University of Tennessee Press, 2011). Here are two reviews you can check out online to get a sense of the edited collection: (1) (2)