One thing I've said frequently over at IGTF is that learners benefit from a variety of approaches.
With children, and with many adults, the most important thing is to get them talking, and the best way to do that is to use topics that are relevant to their daily lives. There are a lot of books and learning programs out there today that take that approach, including the two I mentioned: "Now You're Talking/Irish On Your Own" and "Enjoy Irish." These books take a conversational approach to the kinds of situations that anyone, urban or rural, might encounter: Talking about your family and where you live, discussing jobs, holiday plans, giving directions, etc.
Some adults, however, do best with a very methodical, heavily grammar-based approach, and that's a niche Ó Siadhail's book fills very well. There are also other texts that take this approach, including "Progress in Irish" and Éamonn Ó Dónaill's "Essential Irish Grammar."
I think there's room for both approaches. In fact, most of the learners I know personally have a wide variety of books, computer programs, and audio methods, and find something useful in each.
I'm not sure that Ó Siadhail needs to be updated, when you consider that it focuses exclusively on a particular dialect, and in the area where that dialect is spoken, the subject matter is probably still pretty relevant to a lot of people. I don't know that I'd use it exclusively, unless I were very focused on learning the ins and outs of the Cois Farraige dialect, but that's just me...many people who have much better Irish than I do have used it and swear by it.
There are plenty of other methods our there, though, for those who want a different approach, or who want to supplement Ó Siadhail with other materials. We reviewed a lot of them over at IGTF:
http://www.learnirishgaelic.com/productcomparison/Redwolf