Scots lit as dialogue

From Lee Blessing's Independence. Kess, Jo, and Sherry are sisters.

Jo: "It fell about the Martinmas,
When nights are lang and mirk..."
Sherry: (yawning) Swing it, sister
Jo: Sherry.
Kess: Don't mind her. Keep going. You're good.
Jo: I am not.
Kess: Yes, you are.
Jo: What's "lang and mirk" mean? "When nights are lang and mirk."
Kess: Long and murky -- just like it sounds. Martinmas comes in the winter -- so naturally, long dark nights. Go on. Your accent's great.
Sherry: Yeah, great.
Jo: "The carline wife's three sons came hame..."
Sherry: Came hame?
Kess: Sherry. It means home. "Carline" means old, by the way.
Jo: Old? Carline means old?
Kess: Yup. It's there so we know she can't have any more children. See? Everything in a ballad has a purpose. That's why they're beautiful.
Sherry: "That's why they're beautiful." Same old Kess. (Sherry yawns)
Kess: Will you quit yawning?
Sherry: I didn't get much sleep last night. Besides, I always yawn on Saturdays.
Kess: Go on, Jo.
Jo: (Sings) "The Carline wife's three sons came hame
And their hats were o' the birk.
Sherry: What the hell does that mean?
Kess: It means they're dead.
Jo: What?
Kess: Her three sons are dead. They're wearing hats made of birch. "Birk" means birch.
Sherry: Oh, that explains it.
Kess: But there isn't any birch where she lives. And the next verse indicates that while it doesn't grow there, it does grow at the gates of heaven.
Jo: (Beginning to get it) So...
Kess: So they're wearing hats made in heaven. See? They're dead. They were lost in a shipwreck three verses ago. Remember?
Jo: How do you keep all this straight?
Sherry: She doesn't. She makes it up.
Kess: The point is, we're dealing with ghosts here. This poor old woman has three sons, and she sends them all out sailing -- major mistake -- and word comes back they've drowned. Well, she does't want to believe that...
Sherry: Why not?
Kess: So, some time later, they show up -- surprisingly -- and she's wild with joy. My sons are home! She doesn't notice the birch hats.
Sherry: I would notice the birch hats.
Kess: She doesn't. She loves them, and she can't bear to think they're dead. So she welcomes them, and then she sits and watches over them all night long. But just before dawn she falls asleep. And they wake up before she does, and they leave her forever.
Jo: that's awful. I mean it's pretty, but it's awful.
Kess: They can't help it. They have to get back to their graves.
Sherry: Think I'll have an omelet.
Jo: You always have an omelet.
Sherry: It's all I can cook.
Jo: Do you have any more songs?
Kess: Well, here's one about two crows eating a corpse...
Jo: Oh ... not yet. Let me work up to that.

So, Paul, what song was Jo singing? I recognized the Twa Corbies.......