The Third Policeman came out in 1967, a year after he died; the publishers of his day had passed on it, so he told people the manuscript had blown out of the back of a car in Donegal and gone, which is a better story than a rejection letter and he knew it. He'd been a civil servant the whole time, writing columns as Myles na gCopaleen in one paper and novels as Flann O'Brien on the shelf, under a name that wasn't the one on his pay slip either — Brian O'Nolan, born in Strabane in 1911.
At Swim-Two-Birds begins by saying a good book might as well have three openings, unrelated to each other, and then provides them; which sounds like showing off until you read it and find he means it kindly. It sold almost nothing for about twenty years. I have two copies behind the counter; one was loved, the other has never been opened, and I keep pricing the loved one higher and losing money on principle.
Myles na gCopaleen he lifted off a character in Boucicault's The Colleen Bawn — a stage Irishman borrowed back for the Irish Times, where the column ran twenty-six years. (Myles the Pony, roughly, if you push the Irish.)
The pen names weren't a lark, Jun — the Irish civil service of the day barred its officers from publishing on public matters under their own names, so Myles was partly a condition of employment. He stayed in the service for about eighteen years with that arrangement.