Colin Farrell

Colin Farrell, New Girlfriend Heat Up Las Vegas

By Mark Gray

07/02/2007 at 02:15 PM EDT

One sign that Colin Farrell may be mellowing out: He hit Las Vegas this weekend, but avoided the city's famed nightlife.

Instead, the Irish actor, 31, spent his time with his girlfriend of six months, Muireann McDonnell, reportedly a 22-year-old student from Dublin.

Farrell and McDonnell were spotted smooching in their hotel pool on Friday, and dined that night at Morton's steakhouse. The couple also caught the Cirque du Soleil shows O and Love, where they sipped water, not alcohol.

(Farrell, who was once famously candid about his love of a pint, entered rehab in December 2005 for exhaustion and dependency on prescription medication related to a back injury.)

"Colin said he just wanted to have a relaxing weekend in Vegas," a source tells PEOPLE. "He was really into Muireann. He was making sure she was comfortable the entire time."

Farrell, who has a 3-year-old son, James, with ex-girlfriend Kim Bordenave, recently wrapped the drama Pride and Glory with Edward Norton. His rep did not immediately respond to a request for comment.