They are in the same ballpark. Irreverence is treating serious matters lightly, like making jokes at a funeral. Insolence is rudeness or lack of respect toward a person who is a social superior, like calling your boss a buffoon to his face.
A docent is a guest lecturer, or a tour guide. If you feel like being a bit snarky or pretentious, you could refer to your next substitute teacher as a docent.