Gareth Joseph Downes, The Heretical Auctoritas of Giordano Bruno: The Significance of the Brunonian Presence in James Joyces The Day of the Rabblement and Stephen Hero, in Joyce Studies Annual, 14 (Summer 2003), pp.37-73.
[...] This article discusses Joyces acerbic pamphlet as the first of the belligerent sorties that he wrote in his open war against the Roman Catholic Church, and the pervasive and paralysing influence of the bourgeois Catholic morality that it helped to maintain in the contemporary cultural and intellectual life of Dublin. It discusses Joyces reading of Brunos Italian dialogues and how this encounter steeled him in his own struggle with Catholic orthodoxy, and explores his covert employment of Bruno as an heretical auctoritas in The Day of the Rabblement and Stephen Hero. It argues that an historicist examination of Joyces dialogue with Bruno provides an extremely effective means of realizing some of the urgency and offensiveness of his critical engagement with contemporary Catholicism during the 1900s.
In an interview with James Knowlson in September 1989, Samuel Beckett revealed that the only remark Joyce ever made about Dante . . . Bruno. Vico.. Joyce, was that, although he liked the essay (which was written at his own behest and instruction), he thought there wasnt enough about Bruno; he found Bruno rather neglected. [Damned to Fame, Simon & Schuster 1996, p.107], His comments are, to a large extent, justified; and even though the essay was first published in 1929, Joyces estimation of Dante . . . Bruno. Vico.. Joyce remains as a salutary and instructive comment on the treatment of Joyces complex relationship with the writings and legacy of the heresiarch martyr of Nola in Joycean criticism to date. Becketts discussion of Joyces encounter with Bruno and his appraisal of the significance of the doctrine of the coincidence of contraries in the Wake is relatively telegraphic, when  compared to his more expansive accounts of the importance of Dantes system of poetics and the Viconian theory of the inevitability of cyclical evolution, and, in fact, is cribbed largely from J. Lewis McIntyres 1903 study of the Nolan, Giordano Bruno.
In My Brothers Keeper Stanislaus confirms that Joyce was reading Brunos philosophical essays at this time and observes that his growing admiration for Bruno influenced his decision to choose Gordon Brown as a stage name. (MBK, 1957, p.132.) In The Transformation Process in James Joyces Ulysses [Texas UP 1980], Gose makes no reference to the allusion to The Expulsion of the Triumphant Beast in Stephen Hero, but he does argue that Joyce was indeed familiar with this text and with the other Italian dialogues written and published secretly in London in 1584 and 1585: La cena de la ceneri (The Ash Wednesday Supper), De gli eroici furori (The Heroic Frenzies), Cabala del Cavallo Pegaseo (The Cabala of the Horse Pegasus), De la causa, principio e uno (Cause, Principle, and Unity), and De linfinito universo e mondo (On the Infinite Universe and Worlds). [Gose, op. cit., p.3.] The catalogue of the National Library confirms that there was a considerable collection of works by, and relating to, Bruno which would have been  available to Joyce at this time. The majority of these holdings are listed as being catalogued from the 1890s onwards, and thus the collection reflects the contemporary growth of Brunonian studies in Europe.
Bruno exhibits a modern concept of authorship that is self-legitimating [...] Bruno constructs a mode of authorship that is not dependent on the intellectual and religious discourses of orthodoxy for its signification [quotes Rivka Feldhay & Adi Ophir, Heresy and Hierarchy: The Authorization of Giordano Bruno, in Stanford Humanities Review, 1, 1 (Spring 1989), p.126:] The bizarre composition of the Cena, its discursive layers, and textual practices, may serve as clues to a specific discourse, irreducible either to science or Hermeticism. What Bruno has to offer is a modern concept of authorship, which [constrains] the way he observes and interprets nature, reads and uses texts, the autonomy of his discourse and its potential institutionalisation within a political environment (here p.62).