In the novel The Great Gatsby, F. Scott Fitzgerald wants the reader to become 'one' with Daisy, Tom, Myrtle and George Wilson, Jay Gatsby and Nick Carraway. True to Fitzgerald's talent the reader does identify with each of these characters, however it is Nick Carraway who demonstrates consistent clarity. The reference to 'Castle Rackrent' suggests that Fitzgerald was aware of the hiberian tale and the power of its intent. Through Nick Carraway, Fitzgerald is able to express his unsettled opinions regarding the America he saw unfolding around him. The story of 'Castle Rackrent' was a revelant topic for Nick Carraway, because it was revelant to Fitzgerald. Fitzgerald understood the 'theater of America' during the 1920's wasn't real, it was ..theater. Castle Rackrent was Nick Carroway's recognition that the 'society' still holds some sort of power over its citizens.
Nick and Gatsby are awaiting the arrival of Daisy. When she arrives at Nick's cottage she sweeps into the room and asks Nick, "Are you in love with me,' she says low in my ear, or why did I have to come alone?' 'That's the secret of Castle Rackrent. Tell your chauffeur to go far away and spend an hour." Nick replied.
I believe Nick's reference to Castle Rackrent is meant to call up the plot at the end of the short novel by Maria Edgeworth. At the end of the novel the reader is left to wonder about the story of the Rackrent castle's ownership. "After Sir Condy Rackrent's death, Jason and the now-recovered Lady Condy went to court over the title of the estate. Some said Jason would get the land, and others said Lady Condy would win. Thady could only guess the results of the suit.
I think Nick is using a play on words in reference to a literary reference. Daisy couldn't solve the mystery of why she was asked to come to Nick's alone in the middle of the afternoon; she would have to just guess.