I'm with you - actually the only reason I went with Rosetta Stone was because of a family friend who used it and felt very comfortable with the material. I should have my copy here towards the end of next week so I'll be sure to let you know my thoughts
As for the Irish, I'll be interested to see how it's presented. Due to the differences in Irish dialects (and they are indeed quite different), it would seem fairly difficult.
MithShrike, which dialect are the cassettes/mp3s in? Munster, Ulster, Connacht, etc... As an aside, I know there are some really great resources (free) on the net that I've run across that I would imagine could be a great help to you. One of which was a news broadcast where everything was spoken in Irish. I always thought that would be a great way to train your ear.