All I gathered concerning "In the heather's my bed", a Gaelic poem titled "Ha mo leabaidh 'san fhraoch" in the original language is here:
http://chrsouchon.free.fr/hamoleab.htm.
It's about the famous "Gentle Lochiel".
I have included the mention "melody unknown". But, is it really a song? And, if so, who knows the melody?