On the off chance anyone else still follows this thread: this is my translation/reworking to make a singable, more contemporary version. It's not an exact translation, and leaves the last line repeated in Welsh as a coda.

Lisa Lân (Fair Lisa)
Melody: Trad Welsh
Welsh lyrics: Trad
English lyrics: Mike Whitaker

So long have I loved you / Oft have I lain
Wrapped in your arms / Time and again
Lost in your kiss / Like honey so sweet
Lady of mystery / Made me complete

You were my own / my dearest love
Pure as the stars / Burning above
Anguish and pain / Now you are gone
My life you have stolen / Yet still I live on

When I was walking / At break of dawn
The sound of the birds / Made my heart mourn
They rose on dark wings / Crying songs of despair
Oh how I miss you / My Lisa fair.

When I was walking / As the night fell
The sound of the birds / Made my heart melt
They rose on dark wings / Crying songs of despair
Oh how I miss you / My Lisa fair.

Lisa, come lead me / To where you lay
I would lay down / From cold earth not stray
You here with me / Is all that I crave
Together always / In the dark of the grave

Daw hiraeth mawr / am Lisa lân.