Scots and Scots-Irish have gone everywhere, with no regard to how they happen to spell their surname! Cahoon, Calhoon, Calhoun, etc. are commonly thought to come from Colquhoun, which is pronounced the same way.
We do know that in ancient times, the Scots came from Ireland, and also that the modern Irish came from elsewhere, even longer ago. Ancient peoples were always on the move, it seems.