Under the name Flann O'Brien, he published a series of novels that have attracted a wide following for their bizarre humour and Modernist metafiction. At Swim-Two-Birds works entirely with recycled characters from other fiction (and legend), on the grounds that there are already far too many fictional characters in circulation, while The Third Policeman has a superficial plot about an Irish country youth's vision of hell, played against a satire of academic debate on an eccentric philosopher, and finds time to introduce the atomic theory of the bicycle. The philosopher in question, De Selby, is based on Giambattista Vico, who had been a fascination of James Joyce's, and the importance of the bicycle recalls Samuel Beckett. The Dalkey Archive features a character who encounters a penitent, elderly James Joyce (who never wrote any of his books) working as a busboy in the resort of Dalkey and a scientist looking to suck all of the air out of the world. Other books by Flann O'Brien include The Hard Life (a fictional autobiography meant to be his "misterpiece"), and The Poor Mouth (originally written in Irish as An Béal Bocht).
As a novelist, O'Nolan was powerfully influenced by James Joyce. Indeed, he was at pains to attend the same college as Joyce, and Joyce biographer Richard Ellman has established that O'Nolan, fully in keeping with his literary temperament, used a forged interview with John Joyce as part of his application.
As Myles na gCopaleen, O'Nolan published a regular column entitled "The Cruiskeen Lawn" in the Irish Times, usually in English, but sometimes in Irish, and sometimes in Latin. The columns introduce a regular set of characters, such as the "PLAIN PEOPLE OF IRELAND," "the Brother," and "the Da," include a "catechism of cliche," and propose numerous schemes for the improvement of the Irish nation. These pieces have been collected into a number of books with titles such as The Best of Myles and Cuttings from the Cruiskeen Lawn (an example of bilingual humour, which O'Nolan often used, is both in the pen name, which means "Myles of the little ponies," and in the pun of a small bird, the Curiskeen Lawn). O'Nolan had been one of the first proponents of the study of Irish, and yet as a newspaper columnist he consistently satirized Irish nationalists for their zeal. Some of the characters introduced in the "Cruiskeen Lawn" column (in particular The Brother) are explained in The Hard Life.
Flann O'Brien's writing is sufficiently creative that he counts as a major figure in twentieth century Irish literature. Like others whose primary output was periodical, his work has only recently been receiving wide attention from literary scholars.