HEAT barbecue to medium-high heat.
PLACE potatoes in microwaveable casserole dish. Add water; cover with lid. Microwave on HIGH 8 min., rotating dish after 4 min. Meanwhile, cut thin slice off top of each garlic clove, leaving papery skin on clove; discard tops. (Do not cut root ends.) Place garlic on sheet of foil; drizzle with 1 Tbsp. dressing. Wrap garlic in foil.
DRAIN potatoes; thread onto skewers. Brush with some of the remaining dressing. Grill potatoes and garlic 10 to 15 min. or until potatoes are tender, turning potatoes frequently and brushing with remaining dressing. Remove potatoes and garlic from barbecue; cool.
SQUEEZE garlic pulp from skins into small bowl. (See Tip.) Add mayonnaise and mustard; mix well. Add to potatoes and celery in large bowl; mix lightly. Refrigerate several hours or until chilled.
serving size = 1/2 cup (125 mL)