Fairyland is Carcosa.
Change the place names to better lull players into a false sense of security if you'd like. And maybe spruce up the various races of men. White Men become high elves, Green Men are goblins, Blue Men are Andorians or xvarts if you don't like Star Trek races in your D&D, Bone Men are Newhon Ghouls (i.e. no change really), maybe Red Men look could like Coop devils (wait til you're not at work to google that one if you don't know what I'm talking about). Black Men become drow if you want, though I favor the more purplish depictions. I'm still working on the three unearthly colored races (Dolm, Jale, Ulfire). I'm thinking they look weird when visiting Earth, as if light interacts with their skin all wrong. Maybe one of the races shimmer or sparkles with rainbow hues, another has a mirror sheen and the third is invisible (Jale Men = Invisible Stalkers, who are reclassified as fairies).
Imagine the average Carcosa wilderness hex as a green and pleasant land like you see in a TV commercial promoting tourism in Ireland. It's a gorgeous place, it just also happens to have crashed alien saucers, mutated dinosaurs and abominable elvish sorcerers.