You might be thinking of Cinnabar Island. It's the location of the Cinnabar Gym, a popular spot for trainers to battle and learn about Pokémon.
Here's what you can find on Cinnabar Island:
* Cinnabar Gym: The home of Blaine, the Fire-type gym leader.
* Pokémon Mansion: This building has various trainers to battle, including the legendary trainer, Mr. Fuji.
* Seafoam Islands: Located just off the coast of Cinnabar Island, these islands have a variety of Water-type Pokémon.
* The Professor's Lab: You can find the professor who created the Pokédex, and he'll help you analyze your caught Pokémon.
If you're looking for a specific island, please let me know the name, and I'll be happy to provide information about what you can find there in Pokémon FireRed!