You can get it after you complete Pokemon Channel, a GameCube Game, you need to complete the elite 4 before you can get it
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No, you cannot get Jirachi in Pokemon platinum.
You can't catch Jirachi.
Jirachi is a distrubtion pokemon that is not obtainable anymore unless by trading or cheating (gameshark)
there is no jirachi egg trick, but there is a celebii egg trick in Pokemon crystal
There is no jirachi's cave.