Jason Segel, Cobie Smulders and Alyson Hannigan by Alexandra Wyman/WireImage.com Jason Segel, Cobie Smulders and Alyson Hannigan by Alexandra Wyman/WireImage.com

The cast of How I Met Your Mother put the "work hard, play hard" philosophy into action Thursday night as they celebrated the premiere of their spring season and St. Patrick's Day at the Palihouse Hotel in West Hollywood.

Stars Neil Patrick Harris, Jason Segel, Alyson Hannigan, Cobie Smulders and Bob Saget wrapped up a milestone week of filming with pop diva Britney Spears and turned out in their red carpet finery ready to par-tay.

Jameson Irish Whiskey cocktails flowed, Irish-themed hors d'oeuvres (including mini corned-beef-and-cabbage tarts, whiskey wings, and mini shepherd's pies) were passed around, and Segel dished about Brit-Brit's acting chops.

"She has really good comedic timing," he said. "Her improvs were really good. She was into it and very funny."

Segel also discussed his moonlighting as a big-time Hollywood writer when he's not working on HIMYM. "I'm writing the script to the next Muppet movie," he said. "The fact that I have been sitting in front of my computer working on something that I used to love as a kid is really big for me. It's like a dream come true."

Oh, and did we mention, he's single "and looking"? (Go get him, girls!)

Maybe he should take some advice from Saget, who squired around his girlfriend of 19 months, Michelle (yes, Full House fans, Michelle). He also mentioned that he recently had dinner with Mary-Kate and Ashley Olsen in New York, following his Broadway stint in The Drowsy Chaperone.

So the real question is, do the Olsen twins think of him as a father figure since he played their daddy on Full House? "We are great friends," says Saget. "They think of me as a friend not a father figure. John Stamos and Dave Coulier think of me as a father figure, but the girls think of me as a friend."

Meanwhile, Hannigan - who strutted her stuff in a gorgeous Veena Cava minidress that would rival anything Paris, Nicole and Lindsay owns - dished about her best accessory for the chilly L.A. nights. "Petals," she confided. "Petals are a girl's best friend." (For the uninitiated, petals, aka "breast petals," are nipple shields that, um, give a lady some coverage when she can't wear a bra with her outfit.)

She also gave us some scoop on her character this season. "Well, you see much more of Lily and Marshall's apartment," confides Hannigan. "And let's just say they will be getting really big calves this season." Huh? Our girl Aly is fierce when it comes to fashion, but her hints leave us scratching our heads. "All will be revealed," she promised. - Zoe Alexander

The show's first new ep airs Monday, March 17, on CBS (8:30 pm/ET).