I've heard back from one of our harpists, Gwenan Gibbard, who says she's pretty sure that it's in a cerdd dant tune book - Dafydd Roberts, Telynor Mawddwy - that she has somewhere. She'll look tonight. That fits: ceinciau (pl of cainc) are the harp tunes which are used for singing cerdd dant.

Gwenan works (and records) for Sain Records . I see that they've got the bulk of their catalogue available for downloading now (iTunes) with a goal of getting it all up before long.

sian