"O’Donnell, Sylvia, nee O’Connell, born 1895? 1890?, the much-traveled, much-married mother of Odon (q.v.), 149, 691; after marrying and divorcing college president Leopold O’Donnell in 1915, father of Odon, she married Peter Gusev, first Duke of Rahl, and graced Zembla till about 1925 when she married an Oriental prince met in Chamonix; after a number of other more or less glamorous marriages, she was in the act of divorcing Lionel Lavender, cousin of Joseph, when last seen in this Index."
(Pale Fire)

Spanish author Enrique Vila-Matas, in Dublinesca (2010), describes the mental and physical rambles of an ex-editor who is fond of quotations and sees his life as a written text, called Samuel Riba.The editor-narrator has decided to visit Dublin during bloomsday but he realizes that he's forgotten the name of a bridge where a white horse is seen by every person who crosses it. He checks it in his translation of Joyce's Dubliners by Guillermo Cabrera Infante and finds out that, by a misprint, the bridge is at first called O'Connell but next it becomes O' Donnell.  He now checks it in María Isabel Butler de Foley's translation which only mentions O' Connell but where, mysteriously, he is named Daniel O'Connell.  One more trip to his book-case and he gets Joyce's "The Dead" in which there is no reference to Daniel O'Connell, nor any O'Donnell. 

 

This particular item may have caught E.Vila-Matas's attention because he'd read Nabokov's "Pale Fire" and still remembered a certain Sylvia née O' Connell, later married to Leopold O' Donnell. By unfolding his search by focusing on this particular item he'd be making a subtle reference to Nabokov, along with James Joyce. However, only the author himself can confirm such a conjecture.  It's almost impossible that Nabokov would have inserted a reference to Joyce's Dubliners at this point. Unless there's a particular meaning, related to PF, that can be found in ... a white horse?