Welsh folk song was being actively collected in the 19th century, IIRC from what I've read while looking into the background of Lucy Broadwood - she certainly was in correspondence with various people about it, and had books on the subject in her personal collection. As far as solo rather than band performances of Welsh traditional song goes- Chris Jones is well worth hearing - lovely voice, and a great singer (appearing at Whitby again this year, I think). (And, of course, Mary Humphreys also includes some Welsh songs in her performances).