Several sources, including allmusic say MACDOUGALL'S MEN was written by Carolyn "Coco" Kallis, who also sang background vocal on Pidgeon's album.

I've listened to several sound samples and I can only make some minor adjustments to your lyrics:

CHORUS: I didna come for naught; I came to warn:
MacDougall's men are come, my lads. They'll be here ere the morn.
I've travelled hard upon the road, through the rainy night.
Call the men tae arms, my lads. There'll be a heavy fight.

It seems the lass you've taken in is promised tae anither.
She's no who she appears to be. She lied aboot her mither.
She said she was a tinker lass who lost her way sae blind,
And so she's come untae your door. She knew you'd treat her kind....

I see she has won your hairt, and you'll no give her freely,
But for the kindly deed you've done....