06/06/2007 at 10:15 AM EDT
Michelle Trachtenberg has a new boyfriend – and, as befits a love interest of Buffy the Vampire Slayer's little sister, he's a night owl.
Trachtenberg, 21, was seen all over Las Vegas over the weekend with Brandon Hirsch, 26, who is a manager at New York City hot spot Butter.
The couple first stepped out Friday night at the Killers concert at the Hard Rock Hotel, where they kissed in the crowd and Hirsch swept Trachtenberg off her feet as the band launched into their hit "Mr. Brightside."
Later that evening, the two headed to Caesars' Palace's Pure Nightclub, dancing to tunes spun by DJ AM.
On Saturday, they hit the Venetian's Tao Nightclub for the Oceanaut Watch party. "They stayed by each other's sides all night and they were making out all night," says a witness.
A source close to Trachtenberg tells PEOPLE the couple first met two years ago, but got reacquainted about six months ago and have been quietly dating.
While Trachtenberg lives in Los Angeles, "She's becoming bicoastal and spending a lot more time in New York to be with him," says the source. "She wants to be really low-key about the relationship since this is the first time she's dated someone who is not in the business."
Trachtenberg previously dated X2 actor Shawn Ashmore. Her rep did not return calls for comment about the new relationship.
At the Killers concert, Trachtenberg kept mum about her romance, but told PEOPLE: "I love Las Vegas. It's Sin City and there's a lot of fun to be had."