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.]

Gen. 2. The simplified lyric becomes a popular drinking song.

Go here to read the brilliant rest.  I would merely add that any true Irish ballad must be certain to contain at least two of the following elements:

1.   Be maniacally happy.

2.   Be maniacally sad.

3.   Blame the English for everything bad that has happened to the Irish.

4.   Celebrate an Irishman who left Ireland as soon as he was able.

5.   A celebration of the charms of rural Ireland written by someone who would have sooner died than leave Dublin.

6.   Mention the IRA, without mentioning that during the 20′s many Irish said the letters actually stood for I Ran Away.

7.   Be about the death of a beloved pet or child.

8.   Idolize near alcoholism.

9.   Mention Saint Patrick or a leprechaun.

10. Throw in a few Irish gaelic phrases for the singer to mispronounce.