Jirachi was mostly available via Events since Gen 3.
HOWEVER
If you have a Gamecube and the Pokemon Colosseum Bonus Disc(VERY hard to find nowadays) You can receive a free Jirachi via transfer to a copy of Pokemon Ruby or Pokemon Sapphire, NOT Fire Red/Leaf Green/Emerald. You can receive this Jirachi multiple times but not on the same Cartridge, meaning you can have as many Jirachi as you want, as long as you have multiple games that is.
Hope I Helped!