Polly vs. Grace at the Garrison. The entire damned scene, starting with Polly telling Grace they all know about her and drawing her hairpin out. Grace pulls a gun on her, dispensing with the "sweet innocent barmaid" act, which doesn't faze Polly a bit, and she continues telling Grace precisely what she is, a narc, an Ulster girl who hated low people like Polly until she met Tommy. This strikes a nerve with Grace and she puts her gun down, threatens Polly with her fists - to which Polly laughs, replaces her hairpin, and tells Grace to pour them a whiskey, they're more civilized than that.
Polly: But I should tell you this - I will never forgive you, or accept you, or take you in. And it's me who runs the business of the heart in this family. As far as I'm concerned, you're a snitch from the parish, and if you're not gone from this city by tomorrow, I'll kill you myself.