It is the domain of Varuna.
The one-eyed Irish hag Muircheartach inhabited one of these locations. A compendium of old Chinese folklore written by Yu the Great is named for mountains and these places. One of these places is also the residence of an Ugaritic Elohim that fought against the son of Dagan. In Chinese myth, a resident of one of these locations is a dragon who bonked his head into Mount Bùzhōu, forcing Nǚwā to fix its tilt. The god Yam dwells in one of these places, as does Gòng Gōng. A god from one of these places assists in building a bridge to Lanka when Rama fires arrows into his domain. It is the domain of Varuna. The Lokasenna takes place in a structure in one of these realms, which is ruled over by Ran, the mother of Heimdall. It is also ruled by Ran's wife, Aegir. The Greek god of one of these place was also in charge of horses and earthquakes. For 10 points, identify this domain of Poseidon.
The one-eyed Irish hag Muircheartach inhabited one of these locations.
seas or oceans [prompt on "the abyss" or anything similar; accept shipwrecks on the first clue]