In this blog post, Ted Gioia compares Richard Ellmann's famous biography of James Joyce with the more recent (2012) one by Gordon Bowker. It's an interesting piece that provides a host of details about Joyce.
Here Gioia discusses the unrelentingly biographical nature of Joyce's writing:
In truth, Joyce never stopped writing his life story. Stephen Dedalus, his fictional alter ego, reappears in Ulysses, a book that is, among many other things, a continuation of the narrative of A Portrait of the Artist as a Young Man. Joyce downplayed the autobiographical elements in Finnegans Wake, his final and most daunting book, but even here he could not resist filling the text with elements, large and small, from his day-to-day life. In truth, Joyce knew no other manner of writing. Many have puzzled over his strange admission to Harriet Weaver: "I fear I have little imagination"—a remarkable statement coming from one of the most illustrious storytellers of modern times. Yet his quirky process of writing bears out the claim. The building blocks of Joyce’s books were the details of his day, and he constantly jotted down what he had seen and heard, later reworking this raw material into the stuff of his fiction. The kind of wholesale invention of a Balzac or Tolstoy was foreign to his temperament, and it was all too revealing that when he did aim for epic grandeur in rewriting the myth of Ulysses, he had to set the story in his hometown and make himself one of characters. If Joyce had written War and Peace, it would have probably revolved around a brawl in a favorite Dublin pub.
And here Gioia quotes from a paper that Joyce himself presented at University College in 1900, in which he lays out what seems to be a key aesthetic principle guiding Ulysses:
"Even the most commonplace, the deadest among the living," Joyce insisted, "may play a part in the great drama….Life we must accept as we see it before our eyes, men and women as we meet them in the real world, not as we apprehend them in the world of faery."