What do you mean by "American Fiddle tunes"?

Folk music is a fluid tradition with a myriad of regional variants derived from previous sources. The Appalachian tunes and songs have direct ancestors in the tunes brought by the Scots Irish that settled those hills. This has been shown by the many collectors who have painstakingly catalogued thousands of tunes and correlated them with source material. We can call them "American" because they are here now, but to deny their lineage is uneducated. This information available from a number links available through this site and others.
Have fun doing your research!

As far as "spreading beyond Irish American communities", these old tunes are played worldwide by a broad spectrum of ethnicities. They are particularly popular in the Balkans just now and I have German friends that play them regularly.

By the way, are you a fiddle player? (just askin')