The Pleasures of a Futuroscope is a novel -- as noted above, Dunsany's last, in fact; and its first printing was from 2005 -- Hippocampus Press... hence the late date listed in most cases, though it was written half a century earlier. The Jorkens tales, on the other hand, are collections of what are essentially tall tales which Mr. Joseph Jorkens tells in cadging drinks at the local pub... always outrageous, but in such a way that no one can ever actually prove he
isn't telling the truth....
Fifty-one Tales (which has also been published under the title of one of the stories within the collection:
The Food of Death) is a series of very brief tales (usually one or two pages); essentially parables or moral tales, often with a wistful, bittersweet feeling to them, though on occasion quite grim or quite humorous. Not his best collection, but not at all a bad one with which to start.
River Boy: I don't know of any volume where all his short stories are collected into one volume... not even all his fantasy short stories, as he wrote several volumes of them. The early fantasies originally comprised eight volumes, while he also wrote many others scattered throughout the rest of his career (not to mention several fantastic novels or novels about Ireland with either over use of or hints of the supernatural); but one fairly representative collection would be In the Land of Time and Other Fantasy Tales, edited by S. T. Joshi. You can find Ursula K. LeGuin's review of it here:
Lord Dunsany: In the Land of Time and Other Fantasy Tales, edited by S.T. Joshi, reviewed by Ursula K. Le Guin
(I disagree with her concerning
The Gods of Pegāna, incidentally; this remains one of my personal favorites among Dunsany's writings, both for its concepts, its wit, and the deceptively simple charm of its manner.)
However, you can find all of Dunsany's early collections online, and many of them are in print or easily available through second-hand shops on the 'net.