Wow, like loads of help on this. I'm sorry I didn't get back to it earlier.
The third verse goes...
"When dew falls on meadow, and larks fill the night
When the glow from the "griosach" falls on 'half throw, half light'
I'll slip from my casement and then I'll run away.
Then it will not be long, love, 'till our wedding day"
I find the whole of the song with these lyrics more powerfull. However, I'm still not convinced about the "Half throw half light" bit . But this "griosach" stuff makes good sense, giving the "she's gone and died" verse a bit more atmosphere.
Any more ideas / comments ?