But presumably the vendor cannot compel the purchase of a house that has been burned to the ground.
Maybe there is some aspect of contract law I'm ignorant of here.......
The classic position is that, if you have a contract to buy Blackacre, that includes all the buildings, structures, fences, etc standing on Blackacre. Or not standing, if they happen to be burned down/washed away in a flood/swallowed up by a sinkhole. Blackacre still exists; the contract is still good.
Therefore, risk passes to the purchaser soon as the contracts are signed; he needs to insure agains these risks.
However, the standard conditions of sale used in Ireland include a condition that says that vendor is liable to the purchaser for any loss or damage to the property between the date contracts are exchanged and the date of completion. So if there has been a fire the purchaser still has to buy the property, but the vendor has to pay compensation for the damage done by the fire.
But there are some exceptions - the vendor doesn't have to compensate for damage done by reasonable wear and tear during this period; doesn't have to compensate for damage resulting from the vendor's operations of removal from and vacation of the property; doesn't have to compensate for damage done by restrictions or obligations imposed by state authorities; doesn't have to pay for damage occasioned by the purchaser or their agent. Plus the compensation is capped at the total amount of the purchase price; the vendor never has to pay more than that.
So there's some scope their for arguing about how the damage was caused and/or who caused it, and whether the vendor is or is not liable to compensate the purchaser for it. And in a case where you are buying a heritage property that would cost more to rebuild than to buy, you would still be at a loss. For these reasons purchasers may prefer simply to insure the property themselves from the date of exchange of contracts, and the standard contract terms explicitly recognise the purchaser's right to do this.