Regular commenter cminor, at her blog The Minor Premise, reveals to us how Irish ballads come to be:
The Evolution of an Irish Ballad
Being the surmises of a musical amateur who has lately spent entirely too much time online trying to track down folk music lyrics.
Gen. 1. The Irish take on the British in a battle somewhere on Irish soil. Being seriously outnumbered, they are defeated utterly with great loss of life. Anonymous Irish balladeers compose lyrics honoring the courage of the dead, with individual verses devoted to units from each county involved and to fallen leaders. The result is about 40 verses long, though only six or seven are actually remembered by anyone after the debut.
[Alt. Gen. 1. A minor Irish nobleman takes to the hills after a dustup with British occupiers. Anonymous balladeers compose a mercifully brief ditty depicting the outlaw as a romantic hero and emphasizing his revolutionary cred and sheer heartthrobbiness.]