Bit late to be trying to have your tenants sign up now, you should have conveyed the message to them last September for them to do so (as I did). If they did not register I presume that quango would have been in touch by now.
All you can do is ring that quango and ask if your property was registered as you are the landlord.
At the end of the lease you can give another fixed term lease, if you increase the rent you do not need to notify quango 2. Quango 2 is only looking for the 90€ registration fee with details of the rent for a new tenant not for an existing tenant registered in your property with the PRTB. You only need to contact the PRTB for first registration and any problem tenant issues thereafter.