Dia dhuit, a Thuar Ceatha,
I have tried the Rosetta Stone, and I have mixed feelings towards it. If you have done traditional language learning before (you said you've done Spanish and French), then you may not find it the most useful. It is great for vocab, but doesn't explain the grammar. How it works is just pictures with words or phrases attached to them. Good for vocabulary, but a real nightmare when you're trying to understand what the difference between "Tá sé ina chonai in Éirinn" and "Tá sí ina conai in Éirinn" and "Tá siad ina gconai in Éirinn", for example. The course will have gone far enough that you've figured out that sé is 3rd person singular masculine, sí is third sing feminine, and siad is third plural, but what's up with the mutations!? I tried that as my first route into Irish and was completely bamboozled. That, and then the prepositional pronouns which it never explains, just expects you to pickup.
That said, I don't think it's a bad course, if you can afford it. It is good for vocab and pronunciation (if you want to sound somewhat Munster), but if you go forward with it I HIGHLY recommend a grammar book as a supplement, or you'll constantly be wondering about some things that just don't get explained. Buntus Cainte is great also for vocab, but you're going to have the same problem with lack of grammar teaching. I'm studying Irish now, and can help you with books if you like. Let me know what type of approach you want, and if you need an audio course. Though, the best thing you can do is get involved with a group who you like, and if you can manage, get to an immersion program. I'm going this summer to Oideas Gael - http://www.oideas-gael.com/en/
- under great recommendations from my Irish professor and fellow students. It's got a fantastic reputation.
Let me know how I can be of assistance!