Pat the cod dry and season on both sides with 1 tsp. salt total. Transfer to a plate, cover with plastic wrap, and refrigerate for 1 to 2 hours to season the fish.
In a small bowl, combine the olive oil, olives, thyme, and rosemary. Divide the fish and the olive oil mixture among the 4 bags and add a bay leaf to each. Seal the bags, pressing out as much air as possible, and gently massage the fish to coat it all over.
Put the potatoes and 1-1/2 tsp. salt in a 3- to 4-quart pot. Add cold water to cover by 1 inch and bring to a boil over high heat. Lower the heat and simmer until tender, about 15 minutes. Reserve cup cooking water, then drain the potatoes and return them to the pot. Cover and keep warm.
While the potatoes cook, fill a 4-quart pot with 3 inches of water and clip a deep-fat, probe, or instant-read thermometer to the side of the pot. Heat the water slowly over low heat to 140°F, about 15 minutes. Put the bags of fish in the water (the tops of the bags can stick out) and cook, maintaining a water temperature of 135°F to 145°F, until the center of the fish registers 130°F to 135°F on an instant-read thermometer (open the bags to check), 12 to 15 minutes. Transfer the bags to a rimmed baking sheet and let sit for 5 minutes.
Using scissors, snip off one corner of each bag and drain the liquid (not the olives or bay leaves) into the potatoes. Coarsely mash the potatoes, adding some of the reserved potato cooking water, if necessary, to achieve a rustic purée. Season to taste with salt and pepper.
To serve, mound the potatoes in the center of 4 dinner plates. Open each bag and cut down the center perpendicular to the seal to expose the fish. Using a spatula, lift the fish from the bags and place on top of the potatoes. Scatter the olives over the fish (discard the bay leaves), sprinkle with the chives, and serve.
You can prepare the fish in their poaching bags up to 2 hours before cooking; refrigerate.