Luke's Trip to Cloud City
(Situational Irony) Luke goes to Cloud City knowing full well that he is walking into a trap. The trap: Darth Vader and his goons from the Empire.
Luke and Leia's Kiss
(Situational Irony) Luke and Leia share a kiss but are siblings. This is totally unexpected.
Darth Vader
(Situational Irony) Darth Vader is Luke's father, yet he works hard to kill Luke. Normally, fathers are supposed to nurture their sons, not kill them.
Carbonite
(Situational Irony) Han being frozen in Carbonite by Darth Vader and the Empire, far away from his friends, actually strengthens his relationship with Luke, Leia, and his other friends, which is unexpected.
Darth Vader's Presence
(Dramatic Irony) Audiences know that Darth Vader is at Cloud City and is the one pulling many of the strings for the Empire before the other characters do.