Do you know if the previous owners have already registered your dog on fido.ie?
We got our black lab jessie when she was five months old, she was already chipped but not registered on the site, but we were given the reg form along with all the details of her chip. On the form was a sticker with a six digit code which I entered in the site field below the field for chip number. Perhaps you need to get more information from the previous owner.
If the dog is registered with the Irish Kennel Club, you need to transfer ownership there also. We bought a metal tag for our dog's collar with her name and our phone number on it, just in case she gets lost, even though she is never allowed out without a minder!
Enjoy your dog, what flavour is it?