The passage quoted in the question continues as follows:
Our greatest living phonetic expert (wild horses shall not drag it from us!) has left no stone unturned in his efforts to delucidate and compare the verse recited and has found it bears a striking resemblance (the italics are ours) to the ranns† of ancient Celtic bards. We are not speaking so much of those delightful lovesongs with which the writer who conceals his identity under the graceful pseudonym of the Little Sweet Branch has familiarised the bookloving world but rather (as a contributor D. O. C. points out in an interesting communication published by an evening contemporary) of the harsher and more personal note which is found in the satirical effusions of the famous Raftery and of Donal MacConsidine to say nothing of a more modern lyrist at present very much in the public eye. We subjoin a specimen which has been rendered into English by an eminent scholar whose name for the moment we are not at liberty to disclose though we believe that our readers will find the topical allusion rather more than an indication. The metrical system of the canine original, which recalls the intricate alliterative and isosyllabic rules of the Welsh englyn,‡ is infinitely more complicated but we believe our readers will agree that the spirit has been well caught.
James Joyce (1922). Ulysses, chapter 12. Project Gutenberg.
† “A piece of Irish verse; a stanza, a quatrain” (OED) ‡ “In Welsh poetry, a stanza (now always a quatrain) of a certain metrical structure” (OED)
The pseudonym indicates that this is Douglas Hyde, a leading figure in the Gaelic revival of the late 19th century, who published under the pen-name An Craoibhín Aoibhinn meaning “Little Sweet Branch”. Joyce’s description of him as the “greatest living phonetic expert” is likely sarcastic, for, as a child, Hyde had developed his own phonetic system to transcribe the Irish language:
Irish was a language he had to learn viva voce […]. Using symbols and spellings from French, Greek, English, and (later, when he was introduced to it) German, Douglas devised for himself as a study aid a phonetic system from which he could record in writing the useful phrases, wise sayings, and snatches of poetry and song that he was then acquiring daily. “Noreya ve dhoul rotin,” Douglas’s phonetic rendering of nuair bhí an diabhal ró-tinn (when the devil was very sick), a case in point, begins one of the many Irish-language stories he transcribed in his exercise book, using the system he had developed.
Janet Egleson Dunleavy & Gareth W. Dunleavy (1991). Douglas Hyde: A Maker of Modern Ireland, p. 30. University of California Press.
Joyce’s emphasis on “striking” picks out a word that Hyde was over-fond of:
Who were those Celts, of whose race the Irish are to-day perhaps the most striking representatives? (p. 1)
One of the most striking features of their external policy (p. 6)
The complete difference between the names of the Indian, Hellenic, Italic, Teutonic, and Celtic gods is very striking (p. 14)
a land which presents so many and so striking analogies (p. 58)
The classical reader need hardly be reminded of the striking resemblance (p. 111)
parallels […] which seem altogether too striking to be fortuitous (p. 198)
A striking passage in one of Renan’s books (p. 225)
the most striking king that ever reigned in pagan Ireland (p. 246)
to which he receives the following striking answer (p. 248)
A striking instance of how the Ossianic tale continued to develop (p. 375)
one of the most striking figures in both the literary and political history (pp. 419–420)
Douglas Hyde (1899). A Literary History of Ireland from Earliest Times to the Present Day. London: T. F. Unwin.
In the remainder of the paragraph in Ulysses, Joyce continues to allude to and parody Hyde’s works:
‘Those delightful lovesongs’ are The Love Songs of Connacht, a collection of Gaelic folk ballads which Hyde gathered and first published, with his own English renderings, in 1893. Anthony Raftery was an itinerant bard of the early nineteenth century whose poems, extant until then only in oral tradition, were rescued from oblivion and edited by Hyde in 1903. MacConsidine, another collector of Irish folklore, is cited a number of times in the notes to The Love Songs of Connacht. The talk of ‘intricate alliterative and isosyllabic rules of the Welsh englyn’ is a fair sample of the somewhat turgid style of Hyde’s Literary History of Ireland. Garryowen’s poem, which completes the interpolation, is in the form of a curse, a genre of Irish poetry familiar, of course, to Hyde, who includes some examples in another of his collections with the observation that ‘This book would not be complete without one or two of them being in it.’†
Alan M. Cohn (1962). ‘Douglas Hyde in Ulysses’. English Studies 43:1–6, p. 256.
† Douglas Hyde (1906). The Religious Songs of Connacht, p. 267. Dublin: M. H. Gill and Son.
There does not seem to be a consensus on whom Joyce meant by “a more modern lyrist at present very much in the public eye”.
The anonymous “more modern lyrist” could potentially be William Butler Yeats, as Declan Kiberd suggests, or perhaps even Joyce himself.
Sam Slote (2009). ‘Garryowen and the Bloody Mangy Mongrel of Irish Modernity’. James Joyce Quarterly 46:3/4, p. 552.
The parenthetical “wild horses shall not drag it from us” also remains a puzzle. I can only guess that this is a joke that would be clearer if we were experts on the Gaelic revival. Perhaps the joke is that Hyde’s pseudonym was not actually very pseudonymous since his published work has both names: see for example the title page for Love Songs of Connacht.