From Bulbapedia, the community-driven Pokémon encyclopedia.
The Pokémon Colosseum Bonus Disc was only available by reserving a copy of Pokémon Colosseum. This disc contained previews of the game Pokémon Colosseum and the movie Jirachi: Wishmaker. It is also the only way to get a legit Jirachi. In order to get Jirachi, a Nintendo GameCube Game Boy Advance Cable, Pokémon Ruby or Sapphire, and an empty spot in the party is required. With everything connected properly Jirachi will appear and be transferred to Ruby or Sapphire. Jirachi will be at level 5 and will be holding a Ganlon Berry. It will also have an OT WISHMKR and an ID#20043. The process of getting Jirachi can be repeated as long as the game that is receiving Jirachi has not already used this disc. If the game that is being used to get Jirachi has the Berry Glitch, it will be fixed upon using this disc.