The first scene of Act 5 takes place in the graveyard and while the beginning of the scene has some comic relief in the converstations between the two gravediggers and then the 1st Gravedigger and Hamlet, the end of the scene is very dramatic. Laertes first leaps into Ophelia's open grave and proclaims his love for his sister, then Hamlet rushes into the scene and proclaims his love to be greater than 40,000 brothers' love. The irony here is that in Act 3, sc. 1, after the "To be, or not to be..." speech, Hamlet told Ophelia, in line 120, "I loved you not." The second scene of Act 5 has several examples of irony as well. The poisoned tipped sword that was meant for Hamlet killed Laertes, too. Hamlet also uses that sword, as well as the poisoned wine, to kill the one who designed all that means of death for Hamlet - Claudius. Finally, Fortinbras, who used great subterfuge throughout the play to get his army into Denmark to fight to regain the lands his father lost, becomes king of Denmark without the need to lift a blade because Hamlet and the rest of the royal family are all dead.
Act 4 has a great deal of irony in it, too. There is much conversational irony in the exchange of words between Hamlet and his two old friends, Rosencrantz and Guildenstern. There is situational irony between the acts of Fortinbras and Hamlet's own inactivity.