How do you unlock islands after beating the elite four?

Answer 1

The detailed procedure is found here: http://pokedream.com/games/fireleaf/walkthrough/12-cinnabar.php

When you defeat the gym on Cinnabar Island you will meet Bill outside. He will ferry you to Island One to meet Celio. He will also give you a Meteorite and the Tri-pass which will give you access to Island One, Two and Three. You should go to Two Island, where the Game Corner owner loves collecting space items. However he will be too distraught because his daughter Lostelle is lost. Head to Three Island and battle the bullies, head west past the Bond Bridge to go to Berry Forest. Lostelle is deep in the Forest, you need to do some Surfing to find her. You will find yourself back in the Game Corner. Upon giving the Meteorite to Lostelle's father he will reward you with a Moon Stone. At this point you can return to Island One and talk to Celio, Bill will take you back to Cinnabar Island. At this point you can return to Island One thru Three by going to Vermillion Port. If you don't already have 60 Pokemon, make sure you search for new Pokemon and evolve the ones you have to accumulate at least 60 Pokemon.

To be able to get access to Island 4-7 you need to have beaten the Pokemon League. If you have caught more than 60 Pokemon, Prof. Oak will upgrade your Pokedex! Now head to Island One and talk to Celio. He will tell you his machine is incomplete and needs some gems to work. Do not leave Island One as you need to surf east (to the right) and go north to Mount Ember. There Team Rocket has dug a tunnel. Battle them and the tunnel is yours!

You will find a Ruby there. Now fly back to the Pokemon Center and talk to Celio. He will reward you with a Rainbow Pass, which will give you access to the rest of the Islands.

HOWEVER: Island 8 and 9 are not available, unless you unlock them with a Pokemon event at Toys R Us or GameStop. There is no telling when the next one will be.

Now you should head to Island Four. When you enter Lorelei's house there is no-one there. That's because she is in the Icefall Cave. Go there by going down the steps to the right (east) and go north, you must surf. If you are persistent you can find HM7 Waterfall here. You help Lorelei battle team Rocket.

Once you have succeeded, you must head to Island Six to get the second password. Walk right from the Poke Center into the Water Path. From there you need to go to the grassy area and keep going until you reach the Dotted Hole. CUT the door and you will find a Sapphire in this cave. But a scientist will snatch it and try to console you by telling you the Team Rocket's hideout password. Once you know the second Team Rocket password, their hideout building is on Island Five. To be able to get there head to the right once you arrive, stay on the sand area then go to the grassy area. Keep walking and you will find the Rockets Hideout. It is somewhat tough to defeat the Rockets but once you have done so you will get the Sapphire which Celio needs to allow trading with both Ruby and Sapphire!