In Stuart Gilbert's book about Ulysses - he advances the idea that the ballads originated with the 'bareserks' sort of seers , members of the Vikings skilled in ballad making.

Gilbert says they had Viking feasts, where the berserk would go into a trance (under the influence of booze and he had a theme but he would extemporise for days at a time sometimes. Apparently if he was interrupted, he got very violent and killed people.

It was all part of Joyce's vision of Dublin as the omphalos; the belly button of the world.