There's plenty out there, depending on how much you want to spend! I have a fairly basic one with maps but anything outside Dublin is non-existent. Your area may be better. Mine is a Magellan Explorist 210 I think, its very well made, waterproof and can take a few bumps. Its designed for handheld use while hiking etc.

I usually use it alongside the OS maps and it is programmed to use the OS grid refs so its easy to find stuff thats marked on the map, once its marked correctly. Basic models only have a little line on the screen showing your movements and the most direct route to your target. This can be a nightmare with no maps as you can end up at an impassable gorge or river! As the crow flies navigation is certainly interesting though if you've plenty of time to spare :)

If you already have OS maps for the areas your interested in then you could possibly do with the basic Garmin or Magellan ones, the ones with colour mapping are very nice but expensive and I'm not sure how detailed they would be as far as prehistoric remains. You could end up shelling out a fortune only to have to shell out another fortune for the maps anyway.