Just go to the guy in pacifidlog town everyday and eventually you will be able to go and it is eastward from where you are his house
* Every day, Mirage Island is given a random number. In order for you to be able to go there, the number has to match a number from a Pokemon in your party. It's super rare to get a match, and most trainers will never gain access to the island. Just adding to the previous answer (:
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The only way to get to mirage island is by having the the old man in the lowest right house say he sees the island, then just surf left. =D
Mirage island is not in Pokemon pearl.
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Get the grand prize from the lottery in lilycove city and make sure the Pokemon who won was in your party go to pacifidlog town and go into the bottom right house talk to the old guy he will say he can see mirage island! Use surf and go east until you reach the island.
when the code matches your Pokemon's ID number you can get in mirage island. P.S. its extremely rare