We have an old cisterna in the middle of the Casa which now has no water in it! and its about 12 ft from our external walls, all floors which were dirt floors have been tanked with Visqueen 2 inches of polystyrene block insulation and 2 inches of concrete floor on top then tiled there is a 10ft of concrete walkway all round the building and still the walls above the old cisterna get damp, you are on a loser I'm afraid short of injecting every piece of rock with silicon at floor level!!

If the building is of modern materials ie blocks or bricks then OK try injecting silicone (watch out for the boob area though)

Unfortunately if its and old building the walls are likely to be made of local rocks and mud and the silicone will probably not penetrate volcanic rock

Is it an old village house? As you will know most locals don't actually live on the ground floor and it acts as a natural damp proof course ie. the damp doesn't rise to the first floor.
If the moisture is coming from the floor then you could dig out any mud and loose stone until you reach the beadrock, cover with a waterproof membrane and then poor cement over and tile the area. This made our basement floor very dry but the walls are still moist at times. This would be very time consuming but what you could do is dig out all the old grout ( likely to be mud ) as far into the wall as you can ie, about half way across each stone. Then wet the area and fill in around every stone ( point ) with monocapa. This can be purchased at any buliding merchants. Make it up according to the recommendations. It will set hard and prevent water penetrating. You can either leave the stone bare or plaster over it when you are sure there is no water penetration.

You could also try a de-humidifier if the problem is not extreme, you would probably need to keep it running permanently from Autumn to Spring.
I don't know if any of this is useful but contact me if you need any more info. on renovating an old townhouse as I have experience of this and have spent many happy days digging out basements and digging soil from around rocks!
PS. monocapa pointing also adds great strength to your walls, great if the house next door is abandoned and falling down!! Don't listen to any builders who say slap on some mesh and cement, it will just absorb the moisture!!

No problem! I'm always interested to hear about renovations of old properties. Ours is of a similar age but is on two streets ( front and back ) which certainly aids ventilation.
The back to back houses are more prone to damp but with the right measures they should be dry.
20 to 30 % moisture level in the air still sounds high. You are right to take off any old plaster as just traps moisture. If you are backing onto a hill then you may need to take measures to seal off any moisture. Forget silicone injection, it won't work, particularly as the stones will be water logged.

Are you planning to do any work yourself? It's not difficult to do what I suggested, just time consuming, but it would be well worth the effort to keep your place dry.

I'll see if I can get any more info. on this and PM you if I do.

Let me know how you get on! If it is anything like our place the basement is dryish in the summer but once it rains you can actually see the moisture forming in tiny droplets!! The rest of the house is bone dry, mind you!!