ooo 'eck! When you say dry stone wall, do you mean a proper one made without mortar? If so you're in with a chance - cut the ivy back, treat fresh young growth with Brushwood Killer and let it take effect. If it regrows repeat and repeat ad infinitum until you don't get any regrowth. Depending on how big the roots are you might need to get a dry-stone-waller to take part of the wall down, remove the roots and rebuild. Hopefully you won't have to do that as it could be expensive unless you have a friendly neighbour who is good at dry stone walling.
If the wall has mortar, the initial treatment is the same, but you'll definitely have to take the wall down, remove the roots and rebuild if the roots have got into it and dried the mortar out.
We've just had a 12 metre length of brick wall rebuilt because of damage done to the footings by ivy roots