Jim,my information on Tannahill came verbally from my late neighbour, George Kerr, Hill of Fullwoodhead, Beith. These Kerrs were descended from the Kers of Kersland, Dalry.[just down the road] who were counted the oldest family of the name in Scotland. George had his matriculated coat of arms and like myself was keenly interested in the traditional lore of the countryside with his lands marching on the lands of Mill of Beith on which lands are the falls and linn mentioned in the song. The Warlock Craggy stands about one hundred yards North of the falls on the lands of Knowe on the other side of the burn which forms the boundary between Ayrshire and Renfrewshire.
His oral story on Tannahill I found confirmed when I purchased a copy of "Ponts Cunningham" and I believe there is some further information on the subject in a computer article on Lochwinnoch.
When I get back home I will dig out chapter a verse in Pont.
Joe.