Freezing Mashed Potatoes - any way to prevent drying out?
Once you've added the mash to the pie, let it cool completely then add grated cheese and a few cubes of butter. Cover tightly with tin foil and freeze. When you're ready to cook it, defrost it and cook as normal. The cheese and butter will melt into the potato, remoistening it.

You may also want to leave the mince a little wetter than usual so that the liquid can be absorbed into the mash from underneath during cooking.