"Scotland and Northern Ireland have many ancient bonds that have endured throughout the aeons of both recorded history and back into the dark shadows of the misty primaeval. The oldest and least known is that they are closely related geologically, both being made up of tertiary basalt, a type of black igneous rock, and carboniferous limestone, a sedimentary rock with marine origins.
Moreover, Ulster, Ireland's northern-most province, and western Scotland are actually part of the same prehistoric mountain chain, a chain that is millions of years old and that once included the Appalachian Mountains of North America. This geologic connection is quite ironic when one considers that the same stock of people came to live in all three locales in the historic era: the Scots in Scotland, the Ulster Scots in Ulster, and the Scots-Irish, as they came to be known, in America."
E. Estyn Evans, The Personality of Ireland.
UPDATE: There is now an International Appalachian Trail for walkers that stretches across eastern USA, Canada, Ulster, Scotland & western Europe. See the website for the Ulster stretch here...
www.walkni.com/iat/