Complex sentence: Their ancestors were mostly Protestant Lowland Scottish migrants the largest numbers coming from the "Border Reivers" culture of the Scottish Borders, from Galloway, and Ayrshire, although some came from further north in the Scottish Lowlands and also to a lesser extent from the Highlands.
Split into simpler sentences: Their ancestors were mostly Protestant Lowland Scottish people, many being from the "Border Reivers" culture. Ulster Scots are largely descended from colonists from Galloway, Ayrshire, and the Scottish Borders Country, although some descend from people further north in the Scottish Lowlands and the Highlands.