um...I don't think they are any giant island in Pokemon platinum.But if you mean new moon island then you need a members card which you can get a long time ago.Although you can also get it by ar.
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i have beaten the game, and there is no birth island.
there is no fire island on platinum there is full moon island and crsent moon island (were you catch cresilia and darkria) and iron island that is it
in the basement of the giant mart in veilstone city
Iron Island.
in iron island