Sir Furry, this is exquisite -- I am copying over the liner notes for others to see. I hope Other Tink takes a listen, I think she will like it!
"Welsh music from Ceredwen, about a druid prince who hears sounds calling him from Britain, which is being ravaged by Roman army. He sets out on the long journey, to sacrifices himself on the Black Lake (Y Llyn Du) as a messenger for gods."
Also, you can estimate the age of the song from the dates of the Roman army invading Britain:
"The Roman conquest of Britain was a gradual process, beginning in AD 43 under Emperor Claudius and being largely completed by 87 when the Stanegate was established as the northern frontier."