What is ironic about Romeo's soliloquy in light of the news he soon hears?
Assuming you are referring to the soliloquy in Act I Scene 5 (keep in mind, there are several soliloquies in Romeo and Juliet), in which Romeo notices Juliet across the room for the first time, and his knees melt. In his eyes, she is brighter than the torches that surround her, she is like a white dove among crows, and "Did my heart love till now? forswear it, sight! / For I ne’er saw true beauty till this night" (44-53).
He comes across as a bit fickle in the opening act, by the way. He's just be pining over Rosalind, who won't have anything to do with him, and as soon as he sees Juliet--not even having met her*--he is smitten. Rosalind drops from his mind like yesterday's garbage.
* And having not yet met Juliet, how does he know he loves her? She might have halitosis or crippling BO, or be a shrew.... Ah yes. He's 14. ;)
The irony of the situation lies in his discovery, after he has sought out the object of his obsession and wooed her, that she is the daughter of his sworn enemy.