At last. In John Hullah's "The Song Book" 1884, there is a section on Welsh songs. They are ALL taken from John Thomas's book "Welsh Melodies", including on page 325 "Deck the Hall with Boughs of Holly". For every song in the Welsh section it says after the song "Words (translated from Talhaiarn) by Oliphant". Since it is not a translation, the words are Thomas Oliphant's, written to an old Welsh tune. Thomas Oliphant was born in Scotland in 1799, died in (London) England in 1872.
Which volume of "Welsh Melodies" I will find out and let you know.