In Act 1, Scene 5 of Romeo and Juliet, what are the consequences for Romeo and Tybalt of this seeming "sweet intrusion"?
The quote which you refer to is from Lines 91-92 of the scene. Romeo has "crashed" the Capulet party, and when Tybalt recognizes him as a Montague, he is furious. Lord Capulet calms down Tybalt for the sake of keeping peace at the party, but Tybalt says to himself in an aside:
"I will withdraw, but this intrusion shall / Now seeming sweet convert to bitter gall" (1.5.91-92).
For the time being, Tybalt has decided to obey his uncle, but Romeo's intrusion which seems like a good thing to Juliet at this point will become a bad taste in her mouth if Tybalt has anything to do with it.
The consequences of Romeo's choice to go to the party are ultimately life-ending for both characters. Romeo's appearance fuels Tybalt's hate of him and his friends and leads to the bitter fight in Act 3, during which Tybalt is killed. His death, of course, forces Romeo's banishment which then results in his and Juliet's poor decisions at the play's end.