Many years ago (like, about 15) I listened to a recording of Irish tunes my best friend had checked out of the library. There was a chorus of one song that struck me:
"I'll be true unto my love If he'll be true to me."
It was a tale of two sisters that were after the same guy. I checked through the versions of "Twa Sisters" that are here, but I didn't find this song; and besides, this one didn't include the instrumentation of one sister. I believe it dealt with carnage to one of the sheep ... (although that could be a second song on the same record!) and I think the guy got killed?