From revenue website:
"A first time buyer is a person, (or, where there is more than one buyer, each of such persons):
who has not on any previous occasion, either individually or jointly, purchased or built on his/her own behalf a house (in Ireland or abroad) and
where the property purchased is occupied by the purchaser, or a person on his behalf, as his/her only or principal place of residence and
where no rent, other than rent under the rent-a-room-scheme, is derived from the property for five years after the date of the current purchase. "
As long as your relative is not renting the house from you (i.e. renting the entire house) but is merely living there in a caretaker capacity, you definitely qualify for FTB status.