Isn't this called a translation?

"Ar Hyd Y Nos is one of the most widely known and best loved Welsh folk songs. The tune was first recorded in the Musical Relics Of The Welsh Bards (c. 1784). The Welsh words are by the prolific poet John Ceiriog Hughes (1832-1887).
"In English, this song is often set as a lullabye to words by Harold Boulton. These words can be found in many songbooks. The words on the right [i.e., below] are a more literal translation of the Welsh."

All Through The Night

All the star's eyelids say,
All through the night,
"This is the way to the valley of glory,"
All through the night.
Any other light is darkness,
To exhibit true beauty,
The Heavenly family in peace,
All through the night.

O how cheerful smiles the star,
All through the night,
To light its earthly sister,
All through the night.
Old age is night when affliction comes,
But to beautify man in his late days,
We'll put our weak light together,
All through the night.

© Cymdeithas Madog
18 Mawrth/March 2000

(From HERE

~Masato