Annotations to James Joyce's Ulysses/Telemachus/011
011.12-17 O, won't we have . . . coronation day? One form of an English song to commemorate the coronation of King Edward VII. Its words are repeated several times through the book. It is one of many ditties which Mulligan disrespectfully sings to refer to his surroundings or situation. In this case, he may be using "coronation day" as a rough slang for "payday," because the amount could be reckoned in crowns.
011.28 barbacans or barbican: an outwork of a fortified place, as a castle; a defensive outpost of any sort.