Mirage Island is in Emerald, obtainable the same way as in Ruby & Sapphire. Based on set values of Pokémon and the Island having to match. Click here for detailed info
On it, as with in Ruby & Sapphire is a patch of grass and a Berry Tree holding the rare Liechi Berry, which is found nowhere else in Hoenn
And finally, in the grass is nothing but Wild Wynauts, the un-evolved Wobbuffet at levels ranging from Level 5 to Level 50

And thats all that's on Mirage Island