There are many variations to an Irish Four Ball (sometimes called Irish Stableford). Many Irish Four Balls use the two low balls per hole throughout the tournament. A more popular variation calls for the number of scores per hole to vary throughout the round in this fashion:
Holes 1-6: One low ball
Holes 7-11: Two low balls
Holes 12-15: Three low balls
Holes 16-18: All four scores
(Some using the above format eliminate the one-low ball option, and play six holes each using two low balls, three and four.)
Another variation determines scores based on the type of hole being played:
Par 3 Holes: Two low balls
Par 4 Holes: Three low balls
Par 5 Holes: All four scores
Another common Irish Four Ball variation is to stipulate that teams consist of two men and two women.
So there are many variations, but the basics are that teams consist of four, a Stableford system is used, and a pretermined number of low scores per hole make up the team score.