If your essay response is going to focus on a character, then you are doing a character analysis. There may be literary devices that contribute to characterization through. For example, if the character is supposed to be poor and uneducated and an author uses a local color dialect to bring that characterization across, then that is a literary device. The important thing to remember is to analyze HOW the device contributes to the CHARACTERIZATION.
If the character you are analyzing is considered ironic, you must determine what exactly that means. Are his/her actions different from what was expected of him/her? (Situational irony). Are the character's comments ironic? (Verbal irony) Does the character provide information that then provides the audience with knowledge that other characters don't have? (dramatic irony). In the course of analyzing these forms of irony, it would be logical to discuss diction, foreshadowing, use of dialogue, symbolism, etc. which fall under a typical list of literary devices. Ask yourself how these techniques are contributing to the irony of the character.
The links below will give you some more suggestions on how to write these kinds of responses.