I signed on with GaelTalk last year and it was great! All you need are headphones, a computer, and Skype (which is free). It only takes eight weeks to finish the classes but you will learn alot in a very short amount of time. I visited a gaeltacht in Ireland this summer for a couple o' weeks and thanks to GaelTalk, I was able to hold basic conversations with people and (usually) catch the drift of what everyone was saying.
There are audio files and excercises for all of the lessons but the best thing about GaelTalk are the weekly sessions with a native speaker. They will make sure that your pronunciation is pretty much perfect.
It's hard work but it's definitely worth it.