A commenter (Ofelas) drew my attention to this 1964 film of clog dancing in western North Carolina.
Now I have come across this Irish step dancing from about the same time (1963), filmed in Tipperary in Southern Ireland. I am not knowledgeable enough to comment really, but one can imagine that these styles share similar roots. It certainly seems possible that the Scots-Irish picked some of this up from the original Irish and took it to America.
Here is what Wikipedia has to say about clogging in America.