"Remember your epiphanies on green oval leaves, deeply deep, copies to be sent if you died to all the great libraries of the world, including Alexandria?" The Library of Alexandria was famously the greatest in the ancient world. It was also famously destroyed in late antiquity, so Stephen's self-mortifying fantasy is a little over the top. But Joyce actually did give instructions that were only slightly less pretentious.
So important were his early "epiphanies" in his ambition to become a great prose artist that Joyce at one time viewed them as masterpieces, rather than simply sketches. The editors of a webpage on the epiphanies published by The James Joyce Centre note that in 1902, "as he was preparing to leave for Paris, Joyce gave Stanislaus (who was keeper of the manuscript of the epiphanies) instructions that, in the event of his death, copies of the epiphanies were to be sent to all the major libraries of the world, including the Vatican" (emphasis added). See http://jamesjoyce.ie/epiphanies.
For several centuries, from the 3rd century BC to the 1st AD, the Hellenistic library at Alexandria was a great center of learning, visited by scholars from throughout the Mediterranean world. The library preserved every single text it could get its hands on, often by seizing scrolls carried by visitors to the city, returning copies to them, and keeping the originals. According to tradition, the library was destroyed in a great fire, but scholars have not been able to determine exactly when or how it may have happened.