Here are some lyrics for this song I have in 105 Folk Songs of Scotland, Remick Music, 1963. The air is "Baba Mo leanabh, words by K.R. Moffat.

              1. 
     The mirk is gath'ing in the glen,
     The shadows creep up to the brow o' the Ben,
     The sky, but now sae red, sae red
     Grows dim before the night's dull tread.
     The birdies a'ha'e gane to rest,
     Each little warbler in his nest,
     Has hid his head beneath his wing
     And closed his eye and cease to sing.
              2.
     Now comes the bonnie Lady Moon,
     And on my ain bairnie she gently looks down
     And smiles sae kind as if said she 
     "We'll watch the baby you and me."
     Then shut these twa blue e'en, my dear,
     The moon and mither baith are here,
     We'll guard you weel, We'll guard you
     And hush-a-bye will be our sang.

I cannot attest to how old or authentic this these words are.