3 things 1. What do people mean by you can get Jarich and Dyoxsy via..

1.When people say "you can get Jirachi and Deoxys via Gamecube", that means that,you get Jirachi and Deoxys from a Gamecube,if you have one.If you do have a Gamecube,you can get those Pokemon through the Jirachi bonus disk,that comes with Pokemon Colosseum.You can buy just the bonus disk from EBgames for $8.

2.Make the Eevee like you alot.Check by talking to a girl in Lavaridge town.Give your Eevee items to hold,use it in battle,don't use Bitter items on it and,use items on it to make it like you more.Now,make sure it's night time(it's easier to do this in Gold/Silver because you can tell when it's night)(check the clock in your house to see what time it is)then,make Eevee grow a level.It should evolve into an Umbreon.If it doesn't or,it evolves into Espeon,then it either doesn't like you enough or,it is day time.So,if you don't want Espeon,save before you make it evolve so,you could restart your game and wait until night.If you know what Umbreon and Espeon look like,then you could cancle the evolution if,you know it's evolving into Espeon,by pushing B until it says the evolution is cancled.

3.To get to Mirage Island,you need to have a Pokemon with a Mirage Island ID.To do that,you need to keep checking with that guy in Pacifidlog town a few times a day because,your Pokemons ID's change every day and,Pokemon in the PC may have an ID that may let you see Mirage Island.But,you shouldn't be to excited about getting to Mirage Island.All their is there is,ONLY Wynaut and a rare berry.If you have an AR though,there is a walkthrough that has a code to get to Mirage Island.But,like I said before,Mirage Island isn't as great as every hopes it would be.