Vermillion City (The port), the same spot where S.S. Anne was previously.
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The only way you can get to one island is with either a tri-pass or a rainbow pass (you get the rainbow pass after you beat the elite four as well as the tri-pass if you didn't already have it) but as soon as you do you go to Vermilion city and you are able to pick from islands one to three with the tri-pass but with the rainbow pass you can pick from one to seven islands! As long as you've been on the boat before to get something from someone. Just never mind that.
You get the Rainbow Pass from Bill in order to have access to the other four islands in the sevii islands.
its on island 4. but to get there you have to have the rainbow pass then board the vermillion boat port.
Islands 4 to 8 are unlockable by obtaining the National Dex and recieving the Rainbow Pass from Celio, beyond that all the other Islands cannot be reached as their events have long since expired.
You can only get to Islands 8 and 9 in Pokémon FireRed through the use of cheating devices or through events which will give out the tickets which are needed to access the Islands however the events are no longer available.