As I understand, registration has two meanings. When you first buy a SIM, the vendor registers the SIM with the network whose SIM it is to your name, SSN or some other ID and activates it. The two are done simultaneously in the U.S. and many other countries.
The other meaning of registration is what happens when your phone is turned on, after being powered off, or your SIM registers itself with another network with which your networks has service agreement, provided the network is compatible with the technology in your phone.
I suspect, when the seller said the SIM has been activated but not registered, it's the first kind of registration he is talking about.
In some countries, the vendor collects your ID information, activates the SIM, subject to approval of your registration by the provider. The provider checks your credentials in whatever way prescribed in the country and makes the final decision on whether to continue your service or not. If the information received by the network provider is judged to be incomplete or irregular, your service is turned off by the provider, in which case, you or the vendor have to resubmit your credentials.
The advice regarding putting another SIM etc, will not help you at all, if the SIM has no current registration, or if international roaming has not be turned on.
The best and the only correct advice is that you should contact the provider in Ireland.