Well, you may say it is nonsense but I have a theory that these garbled lines once meant something and that the meaning was lost in translation from another language or dialect. These gibberish fillers may well have at one point been phonetically translatable but now there is no chance as too many years have passed.
Could 'musha ring umma do umma do' at one time been a bit of Gaelic Irish or did the writer simply run out of words, or is it just something that has the right sound about it.
Its lines like this that might give folk music a bad name in some social groupings - I heard an uneducated English chap refer to it as Hey Nonny Noo music - sounds more like a reference to Much Ado About Nothing than folk music 'Into Hey nonny, nonny..' My dad calls some types of folk music 'diddle-dee-dee' but thats a direct reference to the sound of some of the bands and is onomatapaeic.
Anyway unless someone comes with the goods we can specualte forever. I think too you'll find that 'musha ...' etc. is not even in all versions of that song.