The correct response to why the italicized lines in the first paragraph are ironic is:
The reader knows something that Janelle does not.
This is because Janelle mistakenly believes that the new supervisor is starting that day, while the reader (and her coworker, Mark) knows the supervisor is actually starting the following Monday. Janelle's frustration with the subway is directed at a situation that wouldn't have been necessary had she known the correct date.