The Lily of the West is in Colm O Lochlainn's Irish Street ballads, which first came out in 1939. She's called Molly, and the plot's similar to the DT version, except i's not clear whether he actually kills the fella

I stepped up with my rapier
and my dagger in my hand
And dragged him from my false love
and boldly bid him stand
But being mad with desperation,
I swore I'd pierce his breast
I was then deceived by Molly O,
the Lily of the West

Then he's sent for trial, and she testifies against him, but he gets off:

"A flaw in my indictment found
and that soon had me freed

The judge is on his side it seems:

That beauty bright, I did adore,
The judge did her address
"Now go, you lovely Molly O,
the lily of the West.

Colm O Lochlainn found the words in a printed ballad sheet, and he gives it the the tune used for Star of the County Down and My Love Nell.