Ironic film
This film can be seen as a parody of early horror films, and therefore there are a lot of ironic situations. For example, the fact that Janet and Brad arrive at a large house that looks haunted is an ironic depiction of the setting of many horror films.
Frank N Furter irony
Frank N. Furter ironically gets mad at Rocky and Janet for cheating. However, he also cheated on Rocky with Brad.
Brad and Janet's innocence
Brad and Janet are depicted as being incredibly innocent at the beginning of the film, humorously so. However, as the film progresses they are corrupted by Frank N. Furter, and ironically lose the innocence they prided themselves on at the beginning of the film.
Irony of progression
The Rocky Horror Picture Show at the time was seen as an "outrageous" and progressive film. However, nowadays the film has ironically been criticized for being outdated.
Frankenstein
The Rocky Horror Picture Show can be seen as an ironic adaptation of Frankenstein by Mary Shelley. It takes a lot of typical tropes from the horror genre and depicts them in a humorous way.