Flann O’Brien is the best-known pseudonym of the post-modernist Irish author Brian O’Nolan. O’Brien, whose energetic style and creative imagination were praised by James Joyce, Anthony Burgess, John Updike, and many others, is mostly recognized for his novels “At Swim-Two-Birds” and “The Third Policeman.” This month, the Dalkey Archive Press (which takes its name from one of O’Brien’s novels) is releasing two volumes of the author’s less-noted works. “The Short Fiction of Flann O’Brien” includes a selection of the humorous, experimental stories that O’Brien published under a variety of pen names in newspapers and magazines (some are translated into English for the first time). “Plays and Teleplays” is a collection of O’Brien’s works for performance for theatre and television, several of which have never appeared in print before. The editors of “The Short Fiction” call their collection “an initial act of recovery rather than a completist project,” but, taken together, these two editions comprise a significant chunk of O’Brien’s scattered œuvre.