Answer: They are an example of dramatic irony because they announce Romeo's death on account of love.
Explanation: Just to elaborate a little on the answer, it can be added that dramatic irony, which is characteristic of tragic plays, occurs when the reader is aware of an event that is going to affect a character, but the character is neither aware of it nor of its — often tragic — consequences, and acts contrary to what he or she is supposed to. Although Mercutio is mistaken about Romeo's feelings, since he is not in love with Rosaline anymore, he is right when he says that Romeo "is already dead" on account of love, and he is now afraid of his determination to accept Tybalt's challenge. Romeo, nevertheless, will follow his heart, and, by doing so, he will keep the reader attentive and engaged.