Yes. We could, for example, say "Edison's most famous achievement was the development of the light bulb, but he had many other achievements as well. Another of his achievements was the development of the phonograph. Some his biggest achievement was anot ny single invention, but the creation of the industrial research laboratory."

Yes, it is countable in this phrase. An achievement is something that we achieve (have achieved, will achieve, etc.). The article "a" (not "an") is used since the *next* word ("remarkable", not "achievement") begins in a consonant sound.

It's an achievement I believe

Yes it is. If "achievement" is used to express a thing that sb has done successfully, especially using their own effort and skill, then it's a countable noun.