The legendary Pokemon Jirachi was unobtainable until the release of Pokemon Coliseum. A bonus disc for the GameCube was included that allowed you to download Jirachi to your Gameboy Advance.
Only available in Pokeom Coliseum and then on top of that he is only a special event within the game or you can cheat with Game Shark and other cloning.
You can only obtain Jirachi in a special event. Jirachi can only be placed in a 3rd generation game. To get it on a 4th gen game, transfer it through Pal Park.
According to online sources, the only way to capture a Jarichi is to either use a gameshark or another method of cheating or you can go to a Pokemon Event.