One diner said: "They were all over each other, they looked as though they couldn't get enough of each other! They were holding hands, cuddling and kissing throughout dinner."
Mary-Kate was first spotted with Dave at Chateau Marmont Hotel's restaurant on December 4 - where they dined with a group of friends including Nicole Richie and DJ Samantha Ronson.
An onlooker told In Touch Weekly magazine: "They drank beer and left together. They looked pretty close."
A representative for 21-year-old Mary-Kate denied the two are a couple, despite the fact that Dave was recently overhead saying he had split from his girlfriend of eight months, Emily VanCamp.
Mary-Kate is following in the footsteps of her twin sister Ashley by securing herself a new man. Ashley was recently romantically linked to 'Sweet Home Alabama' star Josh Lucas.