Both the irony and the parallelism make the moment particularly dramatic and exciting in which Antonio convinces Sebastian to perform the treasonous act of usurping Sebastian's brother King Alonso. The dramatic irony found in the scene makes the audience sympathize with King Alonso's plight and also puts the audience members on the edge of their seats as they wait to see what will actually happen. What's more, since the audience knows that Prospero has brought the king and his company to the island for the moment of Prospero's reckoning, the audience is also eager to see what will happen because we know that King Alonso's death will foil Prospero's plans. Furthermore, the audience sees that Antonios' plans to have Sebastian usurp King Alonso perfectly parallels what Antonio did to Prospero years ago, and this parallelism adds further literary intrigue.
Dramatic irony is a moment when an audience knows more about a character's situation than the character does at that particular moment. During this scene in the play, both King Alonso and his courtier Gonzalo have fallen asleep due to Ariel's magical music and are thus completely unaware of what is happening around them. Ironically, the two most treacherous characters, Antonio and Sebastian, remain awake. As the audience watches Antonio persuade Sebastian to kill and usurp his brother the king, we see and sympathize with the king's plight, making us aware of the depth of intensity and drama of the moment. Plus, since the king is perfectly unaware of his plight, we can easily see that this is a perfect example of dramatic irony, which is serving to create drama within in the scene.
In addition, the audience is already well aware that years before, Antonio usurped his brother Prospero, the Duke of Milan. Antonio also believed he had killed Prospero, but Prospero was aided, managing to survive and start a new life on the island. Hence, here Antonio is again with treasonous thoughts in mind, trying to persuade Sebastian to allow Antonio to kill an even greater power, the king, so that Sebastian can take the crown. We can draw an interesting parallel between Antonio's act of attempting to usurp and kill the king and his act of usurping and attempting to kill his own brother Prospero. We especially see the parallel drawn when Sebastian states how he remembers that Antonio usurped his own brother, and Antonio heartlessly replies:
And look how well my garments sit upon me,
Much feater than before. My brother's servants
Where then my fellows; now they are my men. (II.i.312-315)
This parallel adds dramatic intrigue because it makes the audience wonder, for the moment, if Antonio will be successful this time in killing and thus successful in usurping the king, which is certainly not what the audience wants to happen. Therefore, this parallel helps create dramatic suspense. What's more, we know that if Antonio succeeds in killing the king, Prospero will no longer have his opportunity of reckoning with Antonio, the king, and the king's company, and this realization also adds suspense for the audience.