I don't know the song but I know of it and have heard Len and others sing it enough times.
Len included it on the Topic album 'Wind and Water' and described it:
"This is yet another song I learned from the late John McGrath. There is a modest seduction scene - modest as with most Irish songs, the seduction isn't too explicit. In this case 'When the birds sang sweet, this young man proved his deceit'".

I have always been struck by the song's resemblance to an 'aisling' or 'vision poem' in which the young woman turns out to be 'Ireland' lamenting her troubles at the hands of the invading 'strangers'.
Maybe Martin Ryan knows - if he's out there....
Jim Carroll