The Dochraid's dwelling is where the Dochraid lives. It's a cave located underneath a tree. The exact location of the cave, as well as its surroundings, are unknown.
Merlin, disguised as Dragoon The Great, went to the Dochraid's dwelling to ask her how to free Guinevere from the "Teine Diaga" (the "sacred fire"). When the Dochraid refused to help him, Merlin had to use Excalibur to get her to tell him what he needed (With All My Heart).