I can't remember if I've said this already, but the modern reader has to cheat to fully understand Ulysses. This is not due to Joyce being intentionally difficult or obfuscatory, but simply because of the novel's realism.
To give just one example, the novel is a very accurate linguistic portrait of the English spoken in Ireland in 1916 and therefore its characters use a lot of street cant and colloquialisms that the modern reader is unfamiliar with. Similarly, the current affairs and gossip discussed by the characters is over 100 years old.
Joyce strived for a pure realism (once remarking, famously, "I have no imagination") and therefore did not allow his characters to escape even for a moment the medium of fin de si