The difference between the market value and the amount paid for the house is a gift subject to a reduction for the right of residence. If the child has not previously received a gift then there will be no CAT if it's less than €310,000.
The right is residence may give rise to a CAT liability.
Should get proper advice to reduce exposure. Potentially sell a share to the child for the value of the mortgage and then live in their own house. PPR will apply to child's share of property.