We prefer first to poach the shrimp, then to peel and devein them. That way they maintain their shape rather than curling and flaring into a "butterfly."
Put the rice wine vinegar, mirin, ginger, perppercorns, juniper berries, and cinnamon stick, along with 1 cup of water, in a pot and cook over medium heat, sitrring from time to time, until the sugar is dissolved. Pour the brine into a wide, shallow dish. This brine can be made up to 1 week in advance. Keep refrigerated.
Remove any large ribs of celery from the hearts. Keep the hearts whole and put them into a large pot with the wine, peppercorns, bay leaves, and a generous pinch of salt. Add just enough water to cover the celery. Cover and simmer over medium heat until the celery is crisp-tender when pierced with a paring knife, 25-30 minutes. Transfer the celery to a cutting board. Cut the hearts lengthwise into ½-inch pieces and add them to the pickling brine.
Using the same pot and liquid in which you cooked the celery, poach the shrimp until just cooked, about 3 minutes. Drain the shrimp in a colander. Peel and devein the shrimp and put in a bowl. Pour the celery and pickling brine over the shrimp. Make sure everything is submerged. If there isn't enough brine to cover everything completely, just give the celery and shrimp a turn every now and then. Cover with plastic wrap, refrigerate, and allow the shrimp and celery to "pickle" for about an hour.
Use a slotted spoon to transfer the shrimp and celery to plates. Drizzle with olive oil, season with salt and pepper, and garnish with celery leaves and chopped chives. Serve cold.