Nia Vardalos was there. So was Alanis Morissette, and the guy who played Javier, the gay Dean and Deluca boss on “Felicity.” The El Rey was sold out as Glen Hansard (singer for the Frames) and Markéta Irglová took the stage as the Swell Season.

The thing is, it doesn’t really matter who you are; if you’ve seen Once, you love it, Irglová, Hansard and the music they’ve created. There was something very magical about that film that captured the hearts of people around the world.

That magic was present on the stage, as Irglová, hidden behind her keys, and Hansard, holding that same relic of a guitar from the movie, exchanged coy smiles, approving nods and loving glances through the body of the piano. It should be no secret by now that Hansard and Irglová, despite their 18-year age difference, are dating, and that real life camaraderie adds infinitely to their very special sound.

Having first come about after Hansard and Irglová were approached by the Czech film director Jan Hrebejk for the soundtrack to his upcoming film Beauty in Trouble, the Swell Season project provided such gems as “Lies,” “When Your Mind’s Made Up,” “Leave” and “Drown Out” for the modern musical Once. Each and every one of those quickly beloved songs was performed, along with new songs like “Golden,” surprising covers like the Pixies’ “Cactus” and Pavement’s “I Want My Life to Make More Sense to Me” and a show-stopping homage to Van Morrison with “Into the Mystic.”

What Hansard and Irglová have achieved with the Swell Season is nothing short of brilliant. Their vocal harmonies lend themselves to a new kind of duet, one in which their very distinct and different voices play an endless game of catch-up, enhancing one another by expressing pure emotion alongside, rather than along with, each other.