Re: Registering with PRTB?
this is from prtb.ie:
If your tenant(s) leaves the rented property then it is deemed to be the end of the tenancy. You will need to register with the PRTB the new tenancy within one month of the new tenants moving in. The tenancy registration form is downloadable from this website.
If at least one of the tenants remain in the rented property and the other tenants are replaced by new tenants, then this would be considered an update of the tenancy details. If there is a change of rent you are required to notify the PRTB and when doing so, you are required to advise of any other change of the tenancy details that have arisen in the interim - no fee is payable when providing an update of tenancy details. The tenancy registration form, which is downloadable from this website, can be used for this purpose also.
Looks like you would have to re-register, but not speaking from experience. Have you spoken to the prtb?