Hello Patrick,
Thanks for taking the time to ask your question here, we will try to help with the explanation!
The best people to ask would be the
Revenue Office as they are the ones that administer the rules. They can be contacted through email using their contacts page.
The fractional split in allowances that you mention is basically a 50% of each of the individual totals. So, instead of 1 litre of spirits or 2 litres of non-spirit for example, it is 50% of spirit allowance and 50% of the non-spirit allowance.
Therefore, in your last example, it would be 500ml of whiskey and 1000ml of non-spirit liqueur.
I hope this helps and that you settle in with us on the island of Ireland.
Martin