PIERCE GOES NUTS ON A PAP!

TMZ has learned the L.A. County Sheriff's Department is investigating Pierce Brosnan for allegedly battering a man in Malibu. The incident allegedly occurred outside Casa Escobar last Friday in a Malibu mall about 6:15 PM. We're told Brosnan allegedly committed a battery on Robert Rosen, a photographer. According to Rosen, Pierce was there with his kids, when Rosen began snapping photos. Rosen says Pierce then said, "Why don't you get a real f**king job." Rosen says he then started complimenting Pierce on his Bond roles. A short time later, an enraged Pierce allegedly told the photog, "Why don't you f**k off, mate," and then struck him in the ribs. A witness says the pap instinctively reacted by kicking Pierce -- we're told, in the stomach.