[rmc]

You just say that the laws in force on a particular date apply locally, minus anything that physically can't apply (eg laws about particular locations).

Yep. Or you give yourself a new constitution and say "All laws previously in force automatically come into force, unless they contradict the new constitution". Ireland did that.

Likewise Ireland cleared out a lot of old laws in the 200X's. It was done by saying "Anything pre-1922 is repealed unless it's on this list".

I'm unsure why they kept some laws, like the 1204 law on "Erection of castle and fortifications at Dublin; establishment of fairs at Donnybrook,Waterford and Limerick", but at least now we can refer to it as the Fairs Act of 1204....