DOWNTOWN GIRL: Irish-born, London-based Orla Kiely is set to open her first U.S. store — and showroom space — at 5 Mercer Street in New York next month. “We’re across the street from agnès b., and near Opening Ceremony,” Kiely said during her spring presentation, which was packed with Fifties-inspired shifts, Peter Pan collars and tea dresses with tiny prints. The 2,200-square-foot Manhattan unit is slated to open in mid-October and carry Kiely’s ready-to-wear and home collections. The designer is also readying a capsule collection for Uniqlo that will bow in March. Sales at the company were up 11 percent last year, and it’s on track for 20 percent sales growth this year.