So put the lines together and it's:

Sheila, dark Sheila, what is it that you're seeing?
What is it that you're seeing, that you're seeing in the fire?
I see a lad that loves me - and I see a lad that leaves me,
And a third lad, a shadow lad - and he's the lad that grieves me.

And whatever I am seeing, there's no fearing and no fleeing . . .
But whatever I am seeing, it is not my heart's desire. . . .
Sheila, dark Sheila, with whom will you be roaming?
With whom will you be roaming when the summer day has flown?

A lad there is who loved me - but loves me now no longer,
A lad there is who left me (and oh! his love grows stronger!
But wherever I go roaming, you shall never find me homing,
For wherever I go roaming, I must wander all alone...

Given the 1919 date of original publication, it has to be read in the context of the Great War.