Mermaid Island is a fabled and large island country in the Sea of Dreamland, which is located some distance away from the coast of Dreamland, and is very close to Walrus Island. As the name would suggest, it is inhabited by mermaids.
Not much history is given about this island, other than that it was founded by and named after the human explorer Gerald K. Mermaid.
Princess Bean tricked her fiance Prince Merkimer into sailing on a party barge to this island in hopes of getting rid of him and canceling their wedding, however things became complicated when they ended up accidentally sailing to Walrus Island instead.
Season one episode ten: "Dreamland Falls"
Season two episode two: "Stairway to Hell"
Season two episode three: "The Very Thing"
Season three episode six: "Last Splash"