According to the link below, you are only exempt from paying the NPPR tax if the relative lives there rent free, and you either live there too, or you live no more than 2 km from the property. If those criteria do not apply to you, you owe the NPPR tax, regardless of who lives in the house.
https://www.nppr.ie/Faq.aspx#fk3
I imagine you could prove to the the NPPR folks who lives where by doing the following:
1. Draw up a formal lease agreement between you and the relative, but set the rent to zero.
2. Produce ESB, gas, Eircom, UPC etc bills in their name at that address. Produce bank or An Post statements, or a drivers license that give that address as their home.
3. Produce the above for yourself, giving your own address.
4. Produce copy of your own lease or mortgage agreement stating where you live.
(The Dept of Social Protection and the drivers lisc folks accept the above as proof of residence. I don't know why the NPPR folks would require something else. )
5. Provide a print out of Google maps of the distance between House A and House B to prove that your own home is no more than 2 kms from the other one.
I would imagine that anyone who owns a second home owes the NPPR tax, unless they can prove that they qualify for an exemption. I could be wrong. Why not call your local county council and ask them if its up to you to declare it and then prove then why you are exempt from it? Or is it up to them to come after you for it, whereby you then produce the paperwork showing why you are exempt?
If you are not exempt from it, declare it and pay it as soon as you can. They will find out about you sooner or later, and the fines for non payment are higher than the tax itself.