Creationists have found a few examples of things which they say could not have developed in gradual steps because simpler forms (or "pieces") of certain traits would not serve as a beneficial function in an ancestor, and that it had to form all at once. Eyes, wings, and bacterial flagella are often used. In each case, biologists are able to find examples of simpler forms which are still functional in organisms living today or which may have lived in the past. This is simply a lack of imagination or research on the part of creationists. A simpler form of the bacterial flagellum is found in bacteria that inject material into other cells. Some aquatic life forms feature simple patches of light-sensitive cells, shallow cups, or "pinhole camera" eyes, all of which have some useful function and can evolve into a slightly more complex form. Tree snakes, flying squirrels, flying fish, and some lizards have rudamentary wings. Any creature able to survive a fall from a little higher than his neighbor will have a slightly better chance of passing on his genes. 1% of a wing is better than 0%, 2% is better than 1%, and so on.
"Irreducable Complexity" would apply to any part of any animal which could be shown to be impossible to evolve through a series of small steps: each of which gives an advantage to the animal.
Atheists have no need to answer this: it is the domain of biologists (who are certainly not all atheists - especially school teachers). Having said that, Darwin's theory would be shown to be incorrect if any such example was found. The search began as soon as Darwin's book was published. Despite thousands of biologists worldwide searching for a hundred and fifty years, no such part of any animal has ever been found.
Creationists claim that they have found some - whenever you hear a Creationist claim this, use google and search for "evolution of X": you'll find a host of articles which give theories on how it could have evolved. That goes for the baterical flagellum, the immune system, and all the Creationist favourites.