how do you get to mirage island?

Mirage Island randomly appears in Route 130, you just need to go to Pacificlog Town and talk to the old guy looking at the window to see if the island is there. He will say to you if he can or cannot see the island. You find some rare berries and wild Wynauts there.

There's a lot more to Mirage Island than just random appearances.
Every day, a number between 0 and 65536 is generated. A Pokemon's gender probability or stats of some sort (I'm sorry to say that I don't actually know) have to match that number. This being said, your chances of seeing Mirage Island are about the same as you winning the Lucky Lotto Number in Lilycove. If you have enough time and/or patience, I guess you could switch your team around and go talk to the old man at the window over and over again, if you're really desperate to get there. The only things that are there are some wild Wynaut and a Leichi Berry.

Go to the PACIFIDLOG TOWN, go to the last house at the right and talk to the old man looking at the window. If he says "I can't see Mirage Island today", that means the Mirage Island is not available. But if he says "I can see Mirage Island today", the Mirage Island is there, it is located in Route 130.

Go to a house in Pacifilog Town that is farthest to the east. Talk to the old man in the house who is staring out the window. If he says that he can see the island today, go to Route 130. The island isn't that special, there are only a bunch of Wynaut there. The only thing of value there is the Liechi Berry, which is a very rare berry.

Here first go to lylycove city after you get there make sure you have all legendary Pokemon and go to pacifidlog town talk to that man looking at the window he will say I can see mirage island and go to mirage island on route 130 tada hey if you found a legendary Pokemon regigigas youre lucky yehey

Well, lucky lucky: http://youtu.be/D-EJULtYl0U


More info: http://www.serebii.net/games/mirage.shtml