Dramatic irony is achieved by giving the audience information about the protagonist's future. An example of this is movies that start with the end first, then take you back into the past to see how the protagonist arrived at this place in his life. In this case, the audience doesn't worry about what will happen to the protagonist. Instead, the audience is filled with compassion and dread as we see the protagonist heading for disaster.
The movie Sunset Boulevard begins with the death of Joe Gillis, the protagonist. The story becomes ironic for the audience as we watch Joe struggle to escape the influence of Norma Dresden and write his screenplay, when we know that is never going to happen.
Irony is used to convey character emotions. Consider the man who is walking through the office speaking in a deadpan voice as he tells us that the man in the cubical next to him is a serial killer. The audience is shocked by this revelation, but the character is not. That speaks volumes about who this character is.
You may want a character to display disgust or the thirst for revenge. Use irony to display this by having your character pretend to show gratitude when he are isn't grateful. The word "thanks" can be used in many useful ways in this case.
Humor and irony have a great relationship. Think of the movie "Liar, Liar." A lawyer who lies all the time has a spell placed on him so that he can no longer lie. This happens on the day of his biggest case where lying is his biggest weapon. See the irony---it makes for big laughs.
In "The Anatomy of a Story," John Truby writes, "Satire is the comedy of beliefs, especially those on which an entire society is based. Irony is a form of story logic in which a character gets the opposite of what he wants..."
In a satiric-ironic screenplay, you have a character acting in way he or she believes is moral, but the outcome is actually immoral. The movie "Emma" is an example: Emma is playing matchmaker for her friend; however, she is caught up in social stations, so while her friend is in love with a farmer, a good man, Emma believes that Harriet should be looking for someone in a higher social station. As a result, Emma is acting against Harriet's best interests while trying to help.