Guest
Paddy' Tunney's 'Hedge School' explanation is the one that always rings the truest to me, though your 'mixture iwing one song' addition would explain things - something thrown into an already existing song.
A couple of years ago, tutor/singer, Brian Mullen played a recording of Mrs Tunney's exquisite rendition of this song to a group of students who had never heard it before - at the end of the session they were queuing up to get a copy of it.
Those who haven't heard it I would urge you to seek out a copy (happy to oblige)
Below is an example of Hedge Poetry on overdiive
Jim Carroll   

It was on the banks of a clear, flowing stream
That first I accosted that comely young dame
And in great confusion I did ask her name
Are you Flora, Aurora, or the fame queen of Tyre?
She answered, "I'm neither, I'm Sheila Nee Iyer

Go rhyming, rogue, let my flocks roam in peace
You won't find amongst them that famed Golden Fleece
Or the tresses of Helen, that goddess of Greece
Have hanked 'round your heart like a doll of desire
Be off to your speirbhean," said Sheila Nee Iyer

May the sufferings of Sisyphus fall to my share
And may I the torments of Tantalus bear
To the dark land of Hades let my soul fall an heir
Without linnet in song or a note on the lyre
If ever I prove false to you, Sheila Nee Iyer

Oh had I the wealth of the Orient store
Or the gems of Peru or the Mexican ore
Or the hand of the Midas to mould o'er and o'er
Bright bracelets of gold or of flaming sapphire
I'd robe you in splendor, my Sheila Nee Iyer