Flann O'Brien is the best known pseudonym of Brian O'Nolan (1911-1966) - who also wrote outstanding comic journalism under the Irish name of Myles na gCopaleen.
O'Brien's output of worthwhile work is very slim, but the best is of the first rank:
At Swim-Two-Birds - a novel, published 1939. Utterly unique - dazzlingly brilliant in parts but wholly unsuccessful (excruciatingly dull) in others; it was a critical success (well reviewed) but sold less than 300 copies.
The Third Policeman - a novel written after ASTB in the early 1940s but rejected, and only eventually published posthumously in 1968. It is an unqualified masterpiece.
The Best of Myles - A collection of journalism published by Picador in 1975. The later Myles na gCopaleen collections are greatly inferior.
O'Brien became an alcoholic - drunk from mid morning every day - from the late 1940s, and none of his later work is worth bothering with. But the above are irreplaceable.
At Swim-Two-Birds has some of the funniest passages I have ever encountered - if you respond to the peculiar drollery of O'Brien's language. In that sense, it is the same order of humour as PG Wodehouse (although utterly different in flavour) - everything comes from the exact use of words, and the 'timing' of the passages.
Having said how much I love this - I would find it unsurprising when other people do not like it. You need to be on the same wavelength - to 'tune in'. It is possible that the reader needs some prior familiarity with Irish dialect and national character - but since I had these, I cannot really judge.
However, the novel was 'experimental' and was in fact a collage of various utterly different books and drafts written by O'Brien (including an MA thesis), including chunks of translated Irish legend (by O'Brien and a friend), letters, and other 'found' material. Some of the seemingly endless passages about Sweeney and The Pooka are best skipped. But the bulk of it is so good that I forgive all.
The Third Policeman is very different in flavour - a nightmarish fantasy/ allegory but with some extremely humorous aspects (particularly the footnotes about the 'savant' De Selby). The whole book is astonishingly perfect.
The Best of Myles is fragmentary journalism; but has a lot of highly original, wonderful, surreal stuff in it. I particularly enjoyed the Keats and Chapman parts.
O Brien's life makes for depressing reading. He did all his good stuff before his early thirties, but was given little or no credit for it (with his greatest achievement completely unknown). The accounts of his morose, irritable helpless alcoholism are horrible - despite that Dubliners romanticise intoxication and have made the later O'Brien into 'a character'.
Compared with other first rate Irish writers, O'Brien is much more natively Irish. Almost all of the (many) top ranking Irish writers were Protestant British (e.g. Spenser, Swift, Sheridan, Goldsmith, Shaw, Wilde, Yeats etc.) - probably only Joyce and O'Brien at this level could be considered indigenous (both were raised Roman Catholic - Joyce lapsed but O'Brien remained devout).
Anyway.... if you don't already know O'Brien's work, I would recommend the best as worthy to stand at the highest level in the relevant genres.
If in doubt, start with The Third Policeman.