Mercutio often jibes Romeo for his obsession with Rosaline, as in this moment from Act 2, Scene 4, which simultaneously functions as dramatic irony and foreshadowing:
Alas, poor Romeo, he is already dead, stabbed with a white wench’s black eye, run through the ear with a love-song, the very pin of his heart cleft with the blind bow-boy’s butt shaft.
Mercutio's joke demonstrates his skepticism about Romeo's interest in Rosaline. He is the only one of Romeo's kinsmen to recognize that Romeo's love for Rosaline may be fundamentally superficial and self-indulgent. In Mercutio's opinion, Romeo is not actually in love with Rosaline; in fact, he hardly knows her at all. Rather, he is love with the idea of loving someone. Romeo, to Mercutio, is merely "run through the ear with a love-song," which has led him to believe that he is attracted to Rosaline (despite being unattractive, as Mercutio suggests: he describes her as a "white wench" with a "black eye").
However, Mercutio doesn't realize that Romeo is now in love with Juliet, not Rosaline, and that the love he is experiencing for Juliet is genuine. This is an example of dramatic irony, since the audience is aware of Romeo's new obsession, having observed his interactions with Juliet in the previous scenes. Mercutio's joke has a ring of truth to it, but it also indicates a clear conflict in the play: Mercutio scorns romance while Romeo is irrevocably drawn to it.
Although Mercutio never learns about Romeo and Juliet's relationship, he suspects that Romeo may have had sex with another woman after the Capulets' ball. ("That’s as much as to say such a case as yours constrains a man to bow in the hams," he says to Romeo later in this scene, using a euphemism for sex.) Mercutio seems to perceive Romeo's interest in women as a betrayal of his bonds with other men. In Mercutio's view, romantic love for women weakens men, though Romeo's love for Juliet actually seems to revitalize him.
Moreover, by joking that Romeo has "died" for love, Mercutio is unwittingly prefiguring Romeo's death, which will indeed result from his love for a woman. To Mercutio, a lovesick Romeo is as good as dead, since he appears to have chosen love over male friendship. This assumption indicates Mercutio's cynicism about the value of romantic love, a belief that Shakespeare subtly undermines throughout the play: though Romeo will die for love, his sacrifice will be a noble one.
In Act 3, Scene 2, just after Romeo kills Tybalt, Juliet's nurse confuses her by rushing into her room and raving about the death of an unnamed man. This is yet another instance of dramatic irony in the play, since the audience is aware that the death the Nurse is referencing is Tybalt's, but Juliet is led to mistakenly believe that Romeo has died. "Ah weraday, he’s dead, he’s dead, he’s dead!" the Nurse cries, alluding to Romeo as Tybalt's murderer: "O Romeo, Romeo, Whoever would have thought it? Romeo!" Juliet, confused, asks: "Hath Romeo slain himself?" When the Nurse responds by saying that she "saw the wound"—meaning Tybalt's fatal wound—Juliet assumes that she is talking about Romeo and falls into a fit of grief.
The Nurse's hysterics hint at the confusions and complications to come in the remainder of the play. Romeo, too, will mistakenly believe that Juliet has died (as will the Capulets and the Nurse). Her ranting also provides some comic relief in the midst of tragedy while underlining another key irony of the play. Throughout Romeo and Juliet, adult characters often behave in illogical or absurd ways, in spite of their apparent wisdom and authority. (For instance, the Nurse describes Tybalt as "the best friend I had," though the two never interact in the play, and the Nurse is clearly closest to Juliet.) Ironically, it is Juliet—who's basically a child—who must patiently reason with the Nurse to get her to explain the situation clearly, suggesting that youthfulness is not necessarily opposed to wisdom or maturity.
Romeo and Juliet is frequently cited as a prime example of dramatic irony for its famous conclusion in Act 5, Scene 3. Romeo, believing Juliet to be dead—due to an error of communication between Romeo and Friar Laurence—ends up killing himself out of grief, though the audience knows that Juliet is only sleeping. Unbeknownst to Romeo, Juliet has taken Friar Laurence's sleeping potion to fake her own death and eventually meet Romeo again. Ultimately, the two are reunited, but only in death.
One one hand, this ironic conclusion can be seen as retribution for Romeo and Juliet's unwillingness to follow their families' rules. By defying both their families and the social conventions they have been raised to obey, Romeo and Juliet have seriously disrupted Verona's social order, undermining its core principles: Montagues and Capulets are not allowed to fraternize, and young people (especially women) are given little to no freedom to choose their spouses or to enjoy true, self-directed romance. Thus, Friar Laurence's warning to Romeo in Act 2 about "violent delights"—youthful passion—leading to "violent ends" is proven correct. Romeo and Juliet's deaths are made all the more tragic because they may have been preventable: if Friar Laurence had been able to deliver his letter to Romeo, warning him about the sleeping potion, or if Juliet had woken up a few minutes earlier, the two lovers may have been able to escape together. By rendering that conclusion impossible, however, Shakespeare suggests that restrictive societies will always have disastrous effects on the people they seek to control and confine.
The play's conclusion also serves as a punishment for Montague, Capulet, and Lady Montague and Capulet—Romeo and Juliet's parents—who, unlike the audience, have been unaware of the burgeoning relationship between Romeo and Juliet. (Only the Nurse and Friar Laurence, who serve as true mother and father figures to Romeo and Juliet, are party to the lovers' romance.) Thus, they are all the more shocked and horrified by the consequences of their own actions, which Prince Escalus makes clear to them when he visits the grisly scene: "All are punished," he declares.
In Act 5, Scene 3, Romeo gazes at Juliet, seemingly dead in the Capulet tomb, and marvels at how alive she still appears. This is a clear example of dramatic irony in the play—one that adds gravity to the tragic conclusion:
Beauty’s ensign yet
Is crimson in thy lips and in thy cheeks,
And death’s pale flag is not advancèd there.
The audience knows—though Romeo does not—that Juliet is alive. She has taken a sleeping potion that makes her appear dead, but the slight flush in her face (the "crimson" in her "lips" and "cheeks") should indicate to Romeo that she is still alive. But Romeo's grief overwhelms him. He is unable to read the warning signs clearly, and the impetuous side of his personality takes over, driving him to kill himself with poison just before Juliet awakes from her deep sleep.
Ironically, Romeo instinctively understands that "death's pale flag" has not yet consumed Juliet entirely. It is "not advancèd" in her face, which still bears the marks of "beauty's ensign." In the moment, though, he can think only of his own tremendous sorrow, and assumes that her appearance is misleading. That Romeo is so close to figuring out that Juliet is alive—but is ultimately unable to do so—adds further poignancy to the tragic spectacle of their deaths.