There is a weird rumor...
First, go to sefloar cavern, then have a zigzagoon, and walk for a very long time until it picks up a "wish tag". Then go to the white rock, press a, and it should crack. Then Jirachi shall appear, and will appear randomly, like the latis.
I didn't make this up, and i'm not sure if it works
I have been through the game's data over and over looking for "Wish Tag" and it is not here, it is not in the game. So could every boddy stop making up rumers about it, look at how many rumer Knives made up about wish tag and Jirachi.