The Aurora Ticket
Deoxys is located on the Ninth Island, also called Birth Island. In order to get there, however, you will need an Aurora Ticket. In the past, the Aurora Ticket could be acquired at "Pokemon Rocks America" Nintendo events, which were held in major U.S. cities. However, the last of these events occurred in 2008. Now, the only way to get to Birth Island, and the Deoxys, is to use Action Replay cheat codes. In order to use these Action Replay Codes, you will need a version 3 Action Replay device that is compatible with the Game Boy Advance.
The first code you will need to enter into your Action Replay is "72BC6DFB E9CA5465/A47FB2DC 1AF3CA86"; this is the master code for the U.S. version of "Pokemon FireRed." If you are using the Japanese version of "FireRed," enter the code "8FA79C07 CABE13C8/F0826040 FE465C57." Note that backslashes indicate that a new line of code should begin.
Next, enter the codes "8D671FD9 6F6BEFF2/78DA95DF 44018CB4" and "07CF2578 E43B4EBF." These codes are necessary in order to enable the use of the next two codes: "2782EFCA 4EED1397" and "641B9F5D 3A0D0482." The final two codes provide you with the Aurora Ticket.
Birth Island
Now that you have your Aurora Ticket you can go to Birth Island. Of course, just because you can doesn't mean you should. Unless if you want to re-enter the codes, you can travel to Birth Island only once. Be sure to bring several Ultra Balls whenever you do leave for Birth Island. Save your game and deactivate the Action Replay codes once you reach Birth Island.
You should find a black triangle at the island's center. Stand by that triangle, but don't touch it, and quickly press "A" 11 times. The triangle will begin to glow red and move as you touch it. You will touch the triangle for the last time when it moves to the top of the screen. After you touch the triangle for the last time, Deoxys will appear.
Deoxys
Deoxys is a level-30 legendary Pokemon. Expect him to be extremely fast and to possess very powerful regular and special attacks. You can also expect Deoxys to have weak defensive abilities. Deoxys' special abilities include Snatch, which steals any positive effects from the next attack you unleash, Psychic, which lowers your special defense score, Pursuit, which gives Deoxys a pre-emptive attack whenever you switch Pokemon, and Knock Off, which causes you to lose whatever item you were holding. Fortunately, the Ultra Balls that you brought along should end the battle quickly.