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Insulating the outside of the foundation is a much better idea if you can do it. Insulating the interior of a stone foundation can be risky due to the fact that that like a CMU or cinderblock foundation, they are not as strong as a poured concrete foundation and they rely only on the mortar between blocks and stones to hold them together, which are at risk of freezing and cracking. Here is a page to start with that will explain that better –
How to insulate a cinderblock foundation wall
Insulating the exterior of an existing foundation comes at a much higher cost simply due to the need to excavate, but it does however afford you the opportunity to do a membrane and drainage as well. To do the outside I would propose that you install a dimple drainage matt, along with a weeping tile drained to daylight if you can, and rigid insulation, rockwool or EPS . If you are doing the exterior then you can follow the procedures for exterior insulation here in page about the best way to insulate basements. Let us know if that helps or if you have any follow up questions for clarification.