Ahhh, yawwwwwn!

Good morning, Joe, and good morning Merry.

It's at the end, or constitutes the whole, of a chorus, in many instances.

It can be compared to the 'Whack fol the daddy, O' line in the likes of 'Whiskey in the Jar'

You are right, Joe - in a way, I think.
Different songs have different 'criteria' for using such 'exclaimations'.
Some are 'Mouth Music' as Bugsy says, and some can be 'nonesense', serving only to encourage hearers of the song to keep the rhythm, or the flow, of the song going.

It is also a good opportunity for the singer to take a quick quaff of the Black Stuff, and return on cue ready for the next verse.

In short: There are many reasons for lines like the one above, in songs. Sometimes a research of the song itself can point you in a direction, as certain words in such refrains can refer to something specific, even though it is enveloped in 'nonesense'
Was that any help?

I don't know. I haven't really woken up yet!

B.