Tattie Bogle wrote: So how do people get on with the very long traditional ballads that can have upwards of 30 verses? In Scotland, we call them “The Muckle Sangs”, but there are others within the English, Irish and Welsh canons. They are much loved and revered by a good number of people and often feature in the various traditional singing festivals, much of the the singing being a Capella, so no middle eights, instrumental breaks, etc.
Do people really follow the lengthy story, or are they just hanging on for the next chorus or refrain when they get a chance to sing a few words themselves?

Again, it depends on the singer and the song. People can listen to very long pieces and appreciate them -- remember, the Iliad and the Odyssey were originally oral! I do find it helpful, if one is going to sing a long ballad, to sketch what it's about in advance, to help follow along. And, as I said above, the plot really needs to be fairly linear; if one has to follow side plots, it's too easy to get lost.

And the performance has to be good. I actually think that instrumentation helps in a lot of cases, though it can hurt in others. For one thing, it can cover up when the singer loses the thread of the song. :-) But the singer must be able to give the song a good performance.

I really and truly think that's the key. It's not the song itself (assuming it is coherent); it is the singer. A short song can survive a bad performer; a long one cannot.