The Southern Islands were a group of islands on the planet Bajor.
History and specifics
In the year 2370, there were reports of religious riots spreading throughout the Southern Islands in the wake of Kai Opaka Sulan's death. (DS9 episode: "The Homecoming")
In 2383, Rebecca Jae Sisko was learning about volcanism at school, so Kasidy Yates told her they could visit Volcano Park in the Southern Islands. (ST - Typhon Pact novel: Raise the Dawn)