In Act 2, Scene 4, Mrs. Fainall and Mirabell discuss a plan of Mirabell's. Mrs. Fainall asks about who Mirabell got to play his fake uncle. Dramatic irony comes into play here:
MIRABELL: Injustice to you, I have made you privy to my whole design, and put it in your power to ruin or advance my fortune.
MRS FAINALL: Whom have you instructed to represent your pretended uncle?
MIRABELL: Waitwell, my servant.
MRS FAINALL: He is an humble servant to Foible, my mother’s woman, and may win her to your interest.
In Act 1, Witwoud mentioned how Mirabell's uncle was coming to town and had plans to cheat him out of his inheritance. This moment is when it becomes clear that Mirabell was in charge of things all along. The audience gets the satisfaction of realizing, after the fact, that Witwoud and everyone who thinks Sir Rowland is Mirabell's antagonist is completely misreading the situation. There is an entire group of people (Mirabell, Mrs. Fainall, Foible, and Waitwell) who are in the know, and now the audience as part of that group as well.
This is one of many twists and turns in the play, where dramatic irony is layered on top of itself. The audience is always trying to stay one step ahead of the characters and keep track of who knows what, but characters keep revealing juicy tidbits like this and changing the landscape of the plot. In this instance, the audience is forced to completely reevaluate their interpretation of an earlier scene. As satisfying as it is to be brought into the loop regarding Mirabell's scheme, it is also a reminder to be on the lookout for the plot to turn on its head once more. A wit can turn into a fool at any time.
In Act 3, Scene 6, Foible carefully finds out what Mrs. Fainall knows about Mirabell's plot. The scene is famous for the dramatic irony that unfolds while Foible tries to make sure the information doesn't get into Mrs. Marwood's hands:
Madam, I beg your ladyship to acquaint Mr Mirabell of his success. I would be seen as little as possible to speak to him. Besides, I believe Madam Marwood watches me.
Foible takes care not to reveal the plot to someone who might reveal it to Marwood or Lady Wishfort. She already believes Marwood is on the case. Ironically, Marwood overhears the entire conversation from her hiding place in a nearby closet. The audience, knowing that Marwood is eavesdropping, is invited to laugh when Foible says that she is worried Marwood is watching her. She, of course, means that Marwood has generally been keeping a close eye on her, not that Marwood is literally watching her right now. This is one of many examples in which the audience has the chance to see characters' plans foiled, even while they think they are being clever. Congreve is always making fun of the people who think they are wits and not fools: no one is exempt from being a fool, even a character as favorably depicted as Foible.
Marwood emerges from the closet after everyone leaves (taking a back route to avoid Mrs. Marwood) and delivers a soliloquy. She is essentially talking to herself, but in a way that allows the audience to see what is going on inside her head. The audience is now privy to Marwood's thoughts and motivations, which are somewhat different to what she reveals to other characters. This moment of dramatic irony at once positions the audience in an omniscient position (none of the characters know as much about what is going on as the audience knows) and also reminds the audience that there are probably more revelations to come.
In Act 4, Scene 4, Millamant and Wilfull have a comic scene shot through with dramatic irony. Millamant is reciting poetry, but Wilfull believes she is speaking to him:
MILLAMANT: That foolish trifle of a heart – Sir Wilfull!
SIR WILFULL: Yes – your servant. No offence I hope, cousin.
MILLAMANT [repeating]: I swear it will not do its part,
Tho’ thou dost thine, employ’st the power and art.
Natural, easy Suckling!
SIR WILFULL: Anan? Suckling? No such suckling neither, cousin, nor stripling; I thank heaven, I’m no minor.
MILLAMANT: Ah rustic! ruder than Gothic.
SIR WILFULL: Well, well, I shall understand your lingo one of these days, cousin; in the meanwhile, I must answer in plain English.
The fact that Wilfull doesn't understand the poetry, or even that Millamant is reciting poetry, is designed to make the audience feel wittier than him. The audience has already been clued into the poem by an earlier exchange between Millamant and Mrs. Fainall, in which Mrs. Fainall indicated that the poem is by John Suckling. This was a clever move on Congreve's part. Even audience members who don't recognize the poem right away are well set up to laugh at Wilfull for misunderstanding the situation. As is often the case, this moment starts to set up a dichotomy between Wilfull the fool and all the wise people in the audience. This scene is delivering on the prologue's promise to keep the audience separate from the people being called witless.
But the very move Congreve pulls to make sure the audience can recognize the poem also serves to subtly undermine some audience members' confidence in their wit. Those who relied on Mrs. Fainall's identification of the poem must pretend they knew it all along. Congreve has also suggested that poetry is artful and often performative. For instance, Millamant only curls her hair with love notes her suitors have written in verse. Millamant's halting performance of Suckling's poem gets in the way of her communicating effectively with Wilfull. The entire misunderstanding could have been avoided not only if Wilfull recognized the poem, but also if Millamant stopped reciting it for a moment to have a conversation. What's more, audience members who are too focused on the poem, or too busy congratulating themselves for recognizing it, might also have a hard time following the thread of the conversation. The audience and Millamant are as much the butt of the joke here as Wilfull is.