Hi Yop
Let's be clear - presumably the house was left to your Dad recently? If he sells it immediately, he will be liable to the increase in value of the house from the date of death. If it's sold fairly quickly, the Revenue will probably accept the subsequent sale value as the value at the date of death, so there will be no CGT.
If it's the only substantial gift or inheritance received from your Grandma, then there will be no CAT(inheritance tax) either.
Brendan