You can just complete the Pokedex. Talk to the game designer at the hotel in Lilycove. The game designer will give you a small hunt of a Jirachi. After you catch it, you get out of that prison. Go to Pacifidilog Town and go to the bottom right house. You will see an old man. Talk to him. If he says he cannot see Mirage Island, then you leave your game boy on, but have it on charge mode. After 2 hours talk to him again to see if he sees it now. Be persistant; keep doing it. Surely Mirage Island will appear.
You don't have to talk to the game designer or do anything about Jarachi. You got to Pacifidilog Town and go down from the Pokemon centre and the last hut along (South East) there is a person standing and an old man in the window talk to him and he will say I have not seen mirage island today or i have seen it. If he has seen it then you can visit it.
mirage island is assigned a # each day and the ID of a WYNAUT in your party must have that number or the man in pacifidelog won't see mirage island
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The Mirage Island in the older Generation III games has a very slim of appearing. The Mirage Island randomly generates a number and if it matches the ID of one of your Pokemon, it will appear and accessible for 24 hours. An elderly man facing towards the window in Pacifidlog Town will let you know when Mirage Island appear.
Find a Sandshrew in the desert at Route 111, 113, or the Mirage Tower, and evolve it into a Sandslash at level 22.
No.
There is no "easiest way" just do the normal way. Normal way: Talk to the old man in Pacifidlog Town. Once he see's it go to route 130. Look for mirage island. Note: Mirage island is not that important it only has the rare Leichi Berries and in the grass all you will find is wild Wynat.
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.