I really need to get off my duff and do that AncestryDNA thing, or something similar. I really would like to know if I have any Celt in me. If I do, my guess would be that it would be from some Lowland Protestant Scot who moved to Ireland in the 17th century, and whose descendants came to Pennsylvania in the 18th century.
A branch of my family did just that! They called them "Ulster Scots". Promised a good deal in Ireland by King James after he cleared the Irish nobility out. But the promises were hollow. . .they encountered as much strife and violence as in the contested borderlands of Scotland.
A low-cost way to get into it would be to join Wikitree. I've found nearly every feature I wanted there. It's free and there is a lot of help.