The cast of "How I Met Your Mother" celebrated their return to the tube with a pre-St. Patrick's Day party hosted by Jameson Irish Whiskey last night -- and ET was there to get all the BRITNEY SPEARS buzz from the cast, including JASON SEGEL, ALYSON HANNIGAN, JOSH RADNOR, COBIE SMULDERS, NEIL PATRICK HARRIS and BOB SAGET.

The cast had nothing but rave reviews of the pop star, calling her time on the set "a lot of fun." Britney plays the role of a quirky receptionist who has her own unique personality. Jason Segel (who plays MARSHALL ERIKSEN) told ET that "[Britney] was terrific...hilarious...a pretty talented lady."

Britney, as earlier reported by ET, is guest starring on an episode of the CBS series, which begins airing new episodes on Monday, March 17 at 8:30 p.m.