Dramatic irony is a literary feature that is very prevalent in Shakespeare's play Romeo and Juliet. It's also a literary technique that drives the plot makes the audience or reader feel as if they are part of the action and are privy to information other characters do not yet know. However, there is no dramatic irony in the lines you are asking about. By the time the Friar speaks the lines you are referring to, he already knows that Romeo is no longer in love with Rosaline and is now in love with Juliet. However, there is dramatic irony in this scene, but it occurs at the beginning of Act II, Scene 3.
In the beginning of Act II, Scene 3, Romeo finds Friar Laurence tending to his herb garden, a hobby and passion the Friar takes seriously and is very knowledgable in. In fact, these herbs he knows so much about come into play in a major way later in the plot. Romeo has just come from Juliet's home. This is the morning after they have professed their love for each other after the Capulet ball and have decided to marry. Romeo can't wait to tell the Friar about the change of events, being that the Friar is Romeo's father-figure in the play. The Friar thinks Romeo is still in love with Rosaline. What the Friar doesn't know that we, the audience/reader already know, is that no longer covets Rosaline. He is now in love with Juliet.