Technically you are correct.
However, there may be some circumstances that he is not liable? but I doubt it.
Is he permanently living in Dublin?
Has he rented the house officially?
Is this the only house he owns?
Has he stayed there himself at any point?
The reason for all those questions is that if he stayed there himself (say at weekends) and didn't have a house in Dublin (nor was claiming rent relief for Dublin) he could be considered an owner occupier under the revenue's rent a room scheme.
Depending then on how much rent he was getting he could be entitled to the rental income tax free.
If that was the case, he would not have had to register the property with the PRTB, would still be a 1st time buyer and would neither have to repay the TRS or the stamp duty he saved.