I've not tried growing potatoes in grow bags but I was reading this article that claims it's doable
Here's the gist of the article:
Step 1: Prepare the Seed Potatoes
About a week before planting, place seed potatoes in a warm spot. When the sprouts that form are about 1/4" to 1/2" long, the potatoes are almost ready to plant. Cut large seed potatoes into chunks about 2" wide. Each piece should have at least two sprouts. After cutting the seed potatoes, let them sit at room temperature for two or three days.
Step 2: Prepare the Bag
Use a pair of scissors to cut several drainage holes in the bottom of a 30-gallon black plastic trash bag. Roll down the sides of the bag and fill about one-third of the way up with potting soil. Place the bag in an area of the garden that receives full sun.
Step 3: Plant the Potatoes
Dust the seed potatoes with agricultural sulfur to protect against fungal diseases. Plant the seed potatoes by burying them, eyes pointed up, about 2" deep in the soil. Water well.
Step 4: Add More Soil
When the potato plants get about 6" to 8" tall, it is time to add more soil and straw to the bag. Add enough soil so that just the top few leaves poke through the dirt. As the potato plants grow, continue to unroll the bag and add more soil. Keep the potatoes well watered but not soggy.
Step 5: Harvest the Potatoes
One clue that the potatoes are almost ready to harvest is that the leaves will yellow and the foliage will die back. At this point stop watering and leave the potatoes alone for two or three weeks so that their skins toughen up. To harvest, slit open the side of the bag to release the potatoes.