The last big archaeo survey was performed in the eighties, but that wasn't comprehensive. Some data will be from much earlier.

The OSI do have the nicest looking maps. I just wish they'd do 1:25000. Also there are some things missing that the UK ones have like bridge types, church types and pubs! Mind you in some places you wouldn't see the detail for 'PH' being written all over the map :-)

Northern Irish OS maps provide a different problem. For security reasons there are some deliberate faults on them!