The O'Nolan papers consist primarily of O'Nolan's outgoing and incoming correspondence between 1938 and 1966, as well as typescripts of all O'Nolan novels except "At-Swim-Two-Birds."  The manuscript materials also include typescripts of the unfinished novel "Slattery's Sago Saga" and several short stories, among them "Ireland Home and Beauty." The collection also includes numerous reviews and news columns under O'Nolan's various pen names, including "A Weekly Look Round," "The Column Bawn," and "Cruiskeen Lawn"; manuscript articles, among them "Irish Whiskey Rebellion," "To the Irishman on the Street," "The Insect Play," and "Robinson's Cruise-O!"; and manuscript "sketches" and essays under the O'Nolan pseudonyms Flann O'Brien, George Knowall, and Myles na Gopaleen.

The collection also features manuscript stage plays, including "The Boy from Ballytearim",  "Faustus Kelly", and "Thirst"; and manuscript teleplays, including "Boots and Saddle," "Hullaballoons at Christmas," "The Ideas of O'Dea," "Is TV a Good Thing?," "Playing the Game," "Present Problems," and "Wedding Bells." There are also a large assortment of newspaper and magazine reviews of O'Nolan's plays and novels; book jackets, notebooks, and photographs of O'Nolan; and a number of related books and magazines, including the 1943 Dublin edition of "Faustus Kelly", the German editions of "At-Swim-Two-Birds" and "The Hard Life", and Hugh Leonard's 1965 stage adaptation of The Dalkey Archive, entitled "When the Saints Go Cycling In."