If I've read you correctly - None is what you want. It means they don't have information on their servers about your domain. Now if someone using eircom broadband/email requests this site or sends an email to that domain, eircom will check their servers, nope nothing there, and get the information from the next level of
dns.
It's too early and I have a cold so somebody might be able to describe the process better or with proper terms but that's the jist AFAIK!
Rgds, Dave