It's likely you're thinking of the Seafoam Islands, which are accessible in FireRed and are located off the coast of Cinnabar Island. Here's how to get there:
1. Obtain the Secret Key: You can get the Secret Key from the Seafoam Islands Lighthouse, which is located on the small island in the middle of the islands.
2. Unlock the Entrance: Use the Secret Key to unlock the gate blocking the entrance to the islands.
3. Explore the Islands: Inside the Seafoam Islands, you'll find various Pokémon, including some rare ones like Lapras and Snorlax.
Remember, Cerulean Cave is not in FireRed. It's a location from the second generation games and doesn't appear in the first generation remakes.