The fist-fight between Sean Thornton, oh right, John Wayne, versus Red Will Danaher. Basically what the whole movie is building toward. The fight not only helps Thorton's self-torment over having accidentally killed a boxer in the ring, but it resolves a handful of other issues the villagers of Innisfree were coping with.
And the winner, not Danaher.
While they don't actually do anything, the thinly veiled threat from the IRA comes across as kinda badass. Michaleen certainly seemed to think so.