Irony, simply put, is the opposite of what one expects. Technically, this type of irony is called situational irony. In this sense, then, it is ironic that Romeo informs Friar Lawrence in Scene ll:
With Rosaline, my ghostly father? No.
I have forgot that name and that name’s woe.
The friar had asked him if he had been with Rosaline, the girl with whom he had apparently been so much in love with that he could not sleep, had become withdrawn, and had consistently consulted the friar about. It is ironic that he has, almost in an instant, forgotten about her. One would expect that, since she had been so much in his heart and mind, he would still harbor some thought or sentiment for her.
It is also generally ironic that both Romeo and Juliet
have fallen in love and expressed deep affection for each other, when they are supposed to be enemies because of the feud between their families.