This part of the play that you refer to of course comes in the famous balcony scene, where the two lovers address each other and confess their mutual love. However, before Romeo announces his presence to Juliet, he views her for a while, comparing her appearance favourably to stars:
Two of the fairest stars in all the heaven,
Haivng some business, do entreat her eyes
To twinkle in their spheres till they return.
By comparing Juliet's eyes to stars, Romeo is obviously trying to capture her beauty and the way that he feels about her. If there is any foreshadowing in this scene, I think it must relate to the way that "stars" operates as a symbol of unchanging destiny in the play. Note, when Romeo receives news of Juliet's death, how he shouts to the universe "I defy you stars!" One of the central themes of this tragedy is the way that fate or destiny, captured in the symbol of the stars, will not be thwarted and how the two lovers are "star cross'd." Thus this description of Juliet could be said to be ironic because it compares her to stars, which are the very things that keep them apart.