I thank all for the input, and debating the meaning of "Celtic" or "bluegrass" (or "traditional" or the dreaded "folk") is certainly beyond the intent of this thread. (Though I did figure this was likely, and at least no outright tempers are yet flaring over definitions and such!) I don't suppose anyone could argue that any body of music (or literature or other cultural attribute) can have many and varied roots. I simply enjoy lots of Irish, Scottish, Welsh, Cornish, Manx, music, the huge and varied family of western European music often loosely grouped as "Celtic," and I enjoy the English as well, and the Canadian, Australian and such offspring of it. Naturally as people move around the world they take their music with them, and naturally it evolves over time (in both new and old worlds). I just have a brother who loves the old-timey and bluegrass type of music, and we have both observed there is a good deal of overlap in styles, and sometimes in the actual melody, of some of the instrumental music, and he has asked me, and I would like, to come up with an album or two that might be considered to fall into that overlap, not necessarily any sort of scholarly progression.
So, Guest of 0936 and Teresa, thank you particularly, I really appreciate something specific to look for. (Had not thought of "Old Plank Road" and it is a great album.) Giok and Greg, I will follow up the leads. Any other suggestions anyone?