Chances are that the Pokemon that is swarming is not available normally in the Sinnoh region.
No, it is inpossible to catch Jirachi on Diamon, Pearl or Platinum.
You cant, you must trade it from Diamon or pearl.
Dialga Palkia, Rayquaza,Kygore and Groudon
it is a swarming Pokemon so you have to be lucky to get it. you have catch it at lake 230 in the grass. by Ryan goodwin email me at:Ryan.goodwin7@googlemail.com
With PokéRadar
go to the forest and go into the old mansion place and go into the back garden you run around there and catch one
you cant you can catch it platinum then trade it to diamond
You'll have to revive it from an Old Amber.
You cant capture it in Pokemon diamond.You have to catch in emerald adn transfer it to Diamong through Pal Park.
You cant........................UNLESS you use action replay
there is non
NO you cant catch syther in Pokemon pearl