Bye lily wean, or Ba lily wain is just to say (go to sleep?) pretty baby. Nothing nonsense about that.

But talking about nonsense lines, I was referring to such verses as:

A farmer was plowing his field one day
Riteful, riteful, tibby eye ay
A farmer was plowing his field one day
When the Divil came to him and to him did say
With a rite fol lol, tibby eye ay
Riteful, riteful, tibby eye ay!

Now those nonsense lines ARE nonsense, or that's the only way I can read them, and their function is to present a vocal musical passage purposely without sense, as I referred to earlier, to stretch out the story over a larger piece of music. They are, as I see it, the equivalent of an instrumental break.

Dave Oesterreich