Q&A for How to Get Jirachi in Pokémon

You have to enter it as a cheat code in an action replay, usually in an emulator, but it can be done on an actual GBA.

You trade for one on the GTS or with another player. On the GTS, choose Seek Pokemon; for the Pokemon wanted, scroll down to the bottom and pick "What Pokemon?" and then type Jirachi. If you have what the other player wants, voila, your very own Jirachi.

Action replay is a cheat system sold separately. You can visit their site to purchase one and you basically enter codes into the action replay which cause changes in the game, like turning a Pokemon into a Jirachi, for example. It's not for everyone as these Pokemon are not "legit" and might be blocked for use in trades or with the Pokebank. That said, a lot of my hacked Pokemon made it through the checks. As long as the moves on that Pokemon are legit, and that Pokemon is able to be shiny, it tends to pass the check. A few different checks are done to determine if it's legit but it's up to you to take the risk.

No, the transfer only works with Ruby/Sapphire as these were the only ones released when the Bonus Disc was available.

Unless another event with Jirachi comes along, which is unlikely as Ruby/Sapphire/Emerald have been out for years and the remakes have been out for a while, too, finding someone else who is willing to trade theirs is the only alternative to these. Bear in mind that Ruby and Sapphire both require a wired connection and DS WiFi is no longer available, meaning Ruby, Sapphire, Emerald, FireRed, LeafGreen, Diamond, Pearl, Platinum, HeartGold, SoulSilver, Black, White, Black 2 and White 2 all now require local connectivity. However, in any game after these, Pokémon from people met online should still remain possible to receive.

Yes, you can simply use the GTS in Pokemon XY or ORAS to trade for one. Usually event Pokemon come in a cherish ball and can't be traded over the GTS, but some people have hacked them and introduced them to the GTS. So you'll have to consider whether you mind that it could be hacked.

No. Unless you use cheat codes. There are many methods of cheating to obtain Jirachi but there was no event in fr/lg to get him.

The Bonus Disc Jirachi from pre-ordering Pokémon Colosseum in the US can actually be shiny, though for whatever reason Pokémon Bank won't accept it as being a legitimate Pokémon, so even if you have one it can transfer no further than 5th generation.