I first heard this song from Simon Trevelyan of the Vancouver BC Folk Song Society, and he learned it from the blind NYC singer, Robert Rodriquez. Robert wrote two more verses for it and entered it in a song competition of some sort, which he won. (This all from his telling). Here are his two verses, which he added to the beginning.

I sang my song in glen and heather
To rouse the tribes in time of war
I sang my song to Pict and Druid
Of mythic heroes in days of yore
But when the might Roman legions
Brought fire and sword onto our land
No more was heard the sound of singing
The legions stilled the harper's hand

I sang my song to noble Arthur
And Guinevere his fabled Queen
I sang of songs of knightly valor
And of the hallowed Grail ne'er seen
But when the bloody Saxon reavers
Came plundering Albion's fair shore
They drenched our lives in blood and fire
And darkness filled the land once more

Blessings,
Barbara