I think you need to reconsider your choice of solicitor.
Technically, there is an annual gift if someone lives in a house rent free. The value of the gift is the amount of rent which would be payable in the open market which, in the case of a house inhabited by an elderly woman, I'd imagine is very low. Each year, a gift of up to €3,000 may be received from any person and ignored for tax purposes - so assuming that the value of the "free rent" was less than €3,000 per annum there is no way it would impact on their tax free threshold on the inheritance.
And, as you state that the person was living there caring for their incapacitated mother, then I think it is arguable that the "free rent" was not free at all and there was no annual gift.