Presumably, in this case, the beneficiaries never previously received any CAT group A inheritance(s) - e.g. from another parent - and their full €400K exemption threshold is still available to them?
That;s how it works. If you inherit (or receive a gift of) a 50% share in an asset, the value of the inheritance or gift is 50% of the value of the asset, and CAT is calculated on the value of the inheritance or gift.
Take a slightly more complex (and not very realistic) example in which A leaves a house worth €860,000 to his daughter B and his niece C. Neither B nor C have every received any gifts or inheritances before. No special reliefs are avaiable based on facts not stated.
B gets an inheritance of €430,000, offsets her €400k allowance and pays CAT on €30k.
C gets an inheritance of €430,000, offsets her €40k allowance and pays CAT on €390k.