The e-Reader is a simply a GBA cartridge that can have save files backed up and restored to it.
This save file, once restored using a homebrew like GBA Backup Tool by Rudolph will allow you to get the Eon Ticket then access to Southern Island in Ruby and Sapphire, NOT Emerald. I believe this save file works for all e-Readers except the Japanese one.
The hardware you need to actually obtain the Eon Ticket in the game using this method includes:
- 1x e-Reader
- 1x DS Lite (not original or DSi)
- 1x GBA Link Cable
- 2x GBA gaming consoles (any but Game Boy Micro will need a special Link Cable)
- 1x Slot-1 DS Flashcart