The bits of Deasy's letter that we receive through Stephen's interior monologue are sometimes easily intelligible, sometimes elusive. The point of his "Pardoned...classical allusion" (sorry, it's a way we learned schoolmasters have) to "Cassandra" seems obvious enough: Deasy is a prophet whom no one heeds. But who is the "woman who was no better than she should be," and why is Deasy misogynistically commenting on her virtue?
Cassandra was a Trojan princess, daughter of King Priam and Queen Hecuba, whose beauty inspired Apollo to give her the power of prophecy. But when she refused to reciprocate his love he added the curse of having no one believe her predictions. She foresaw the downfall of Troy, which the Trojans might have prevented had they listened to her. Similarly, Deasy is predicting the downfall of the Irish economy if FMD becomes established in the island, but no one will listen to his warnings.
It seems unlikely that Cassandra is the woman "who was no better than she should be": she refused to fornicate with the god, and her sexual relations after the fall of Troy (first with Ajax, then with Agamemnon) were coerced. A little later in the episode, we learn from Deasy's verbatim repetition of the phrase that he is referring to "Helen, the runaway wife of Menelaus." What is she doing in the letter? Apparently it contains digressions within digressions.
 Thornton notes that "a woman who is no better than she should be" is a proverbial saying going back as far as 1604. He cites several instances.