At the Merriam Theater on Sunday night, the Swell Season went one better. After the audience had mastered the wordless coda to "High Horses," singer Glen Hansard called for them to harmonize, a request that at most shows would have resulted in a wave of unlistenable cacophony. But such was the spell cast by Hansard and his musical partner, Markéta Irglová, that 1,800 backing singers rose to the challenge, their voices blending with nary a false note.

It's been said that the secret to a happy ending is knowing when to stop, which in Hansard and Irglová's case would have been sometime last year, after they'd won an Academy Award for their song from the movie Once, and before their romantic relationship fell apart.

It's easy to read the songs on their new album, Strict Joy, as a chronicle of their fraying ties. "I know we're not what I promised you we'd be by now," Hansard sang on "The Rain," "but maybe it's a question of who'd want it anyhow." But then Hansard, Irishman that he is, has always had a soft spot for doomed romanticism, ably documented on his six albums as lead singer of the Frames, who served as the evening's unbilled backing band.

Although the success of Once brought the duo well-deserved (and, in Hansard's case, long overdue) recognition, the new album feels constrained by their success, more concerned with not disappointing expectations than with challenging them.

But in performance, the record's occasional airlessness gave way to boisterous intimacy. When Irglová stepped away from her grand piano to sing the airy "Fantasy Man," Hansard helped himself to the cup of tea she'd left behind. They may no longer be a couple, but you would have had to strain to see any sign of tension between them.

The two-hour set drew mainly from the Swell Season's two albums, although they made room for songs by Willie Nelson and the Clancy Brothers, as well as one by the opener, Doveman (Thomas Bartlett), who enlivened his own set with a deliberately mournful medley of songs from the Footloose sound track. It's doubtful the piano-driven arrangements, concocted as a birthday present for an old friend, will do for him what Once did for his tour mates. But then, a few years ago, you might not have given the Swell Season strong odds on breaking through themselves.