The first instance of irony in those lines is seen in the phrase "kill your joys with love" (V.iii.304). What this phrase refers to is the fact that Romeo and Juliet fell in love and died as a result of that love. Since their love was separated by their families' hatred, in an ironic way, love kills them as well as hatred. Had Romeo and Juliet never met and never fallen in love, they never would have died premature deaths. In addition, the word "joys" in the phrase "kill your joys with love" of course refers to Lords Capulet and Montague's children, Juliet and Romeo, who, of course, were Lords Capulet and Montague's joys. Therefore, Prince Escalus is saying that God killed their joys, their children, using the love they shared for each other, which is very ironic because normally we don't think of love as causing death.
The second instance of irony in these lines is seen in the fact that God has used the love of Lords Capulet and Montague's children to punish the hatred that Lords Capulet and Montague feel for each other. The reference to punishment is seen in the clause, "See what a scourge is laid upon your hate" (303). Normally, we would not think of love as a form of punishment. However, in this case it is a very appropriate form of punishment as it is their hatred that separates the couple's love, and it is both the couple's love and the hatred surrounding them that leads them to their death.
Hence, the irony found in these lines is that love led to Romeo's and Juliet's deaths, and the further irony is that God used love, which caused death, as a punishment for hatred.