Actually, you can get Jirachi with Pokemon Channel, but only on the European version of the game. If you have a US version, you can't get Jirachi.
To get Jirachi downloaded, first you have to complete the game, this involves watching all the channels at least once, plus the full version of the Pichu Bros cartoon (including the 5 seperate parts initially as well). You don't have to collect all the Nice Cards, which helps.
After that is done, on the title screen you will be given the option to download Jirachi to a Ruby/Sapphire/Emerald cart. You can do this once per cart, and Jirachi will either be holding a Ganlon Berry or a Petaya Berry (I'm sure it's either one of those two, my one had the Ganlon).