Joe’s Pub is home to one of the great rooms in New York City. And on Monday night, it hosted one of the great bands you’ve probably never heard of — though, apparently, far more people than we would’ve expected have. A considerable portion of the sold-out, standing-room-only crowd could likely be attributed to the gangly guy standing stage-right.

One month ago, 23-year-old John Gallagher won a Tony Award for his performance in the breakout musical of the year, “Spring Awakening.”

But playing the hormonally charged role of Moritz Stiefel in a toast-of-the-town stage production is merely Gallagher’s two-day, six-nights-a-week job. It’s an occasional late-night gig that finds the young thespian co-fronting Old Springs Pike, a musical zeitgeist that seems to willfully, and winningly, defy categorization.

Thirty seconds into the opening number, as an acoustic whisper gave way to a roar, the term “thrash-folk” came to mind. It seemed appropriate enough. Gallagher, with his awkward gait and slightly exaggerated features, is vaguely reminiscent of Ben Lee, who Jim filmed for MTV News in this very same space eight years earlier.

“Acoustic music’s always been like the most violent and the most aggressive ‘cuz it’s got that cutting sound when you … hit the strings,” Ben had told Jim then. Nearly a decade later, Old Springs Pike seemed to be proving his point.

Their hour-long set had them moving effortlessly between beautifully melodic numbers evoking everything from Out of Time-era R.E.M. to the Doobie Brothers’ “Black Water.” Less accessible songs had a more combative, almost avant-garde feel. They willingly embrace dissonance, and their execution is impeccable, from their tight, four-part, CSN-worthy vocal harmonies (no “pitchiness” here) to their wickedly intense percussive power, which featured everything from drums and cymbals to empty water bottles.

But here’s the thing: They just make it look so freakin’ fun! You find yourself wishing there was a sign-up sheet. Perhaps it’s because they actually are all friends from high school (they grew up together in Delaware), OSP is exactly what you’d expect if the indie-rock hipster crowd from a school like the one that inspired “Fame” formed a band. And their “we swear we don’t rehearse it” banter between songs makes you wonder why your own friends aren’t this amusing.

If fact, we’d be remiss if we didn’t cite one of OSP’s influences as (stick with us here) the Upright Citizens Brigade, NY’s most celebrated improv troupe. Seriously. Like Upright Citizens, OSP play off of one another like consummate pros. And like OSP, there was a time when Upright Citizens was one of the underground gems of the New York downtown scene (oh, how we miss those days). Now, they are movie stars and “Saturday Night Live” regulars. Old Springs Pike will no doubt forge their own path(s), but seem destined for the same level of success.

Since Jim has decided that he wants to name-check colleague James Montgomery in as many blog posts as possible (kind of like the Beastie Boys’ mission to name-check themselves in every song on License to Ill), he’d like to point out that the only female member of the band, Heather Robb, is aesthetically a lot like James’ beautiful new wife, Sasha. (Garth thinks Heather has much more of an A.J. Langer quality. We could make you look that reference up on Wikipedia, but we’re feeling generous: She played Rayanne on the legendary “My So-Called Life”).