Mix together the horseradish, vinegar, ketchup, paprika, cayenne, salt and garlic.
Gradually add the oil, beating thoroughly.
Stir in the green onion and celery.
Add the cooked shrimp, cover and let marinate in refrigerator for 5 to 6 hours.
Place the shrimp on a bed of lettuce, whisk dressing in bowl and spoon more over the plate as desired.