When director Martin Scorsese looked for the perfect song to blast over the beginning of the Boston-set “The Departed,” he found it in Massachusetts-based Dropkick Murphys’ “I’m Shipping Up to Boston.” The group found that out the same time moviegoers did.

“They never told us how they were going to use the song. We were at the local premiere, and there it was, playing in the opening credits, and playing over it loudly,” bassist and singer Ken Casey said. “We were blown away.”

With songs on films like “The Fighter,” performing at Red Sox and Bruins games, and raising millions for local charities, Dropkick Murphys have stitched themselves into the fabric of Boston culture with an authentic Irish-punk sound that seems all too perfect a fit for often rowdy, sometimes sentimental Beantown residents.

“The Boston roots are cool. It’s fun music to play,” Casey said. “We never had the intention to represent our culture or roots, but we certainly can’t get away from that, and it’s not a bad thing, either.”

Decidedly heavier than The Pogues or Flogging Molly, the seven-piece began as a straightforward punk band with ambitions to inject a little of the Emerald Isle, but limited by their skill.

“We started finding session players to come record with us to add some of that flavor, but we didn’t really have the means to re-create that live,” Casey said. “All of a sudden, we had this army of teenagers learning to play that type of music. Kids we met at shows told us they could play these songs on bagpipes or the mandolin. Next thing you know, they were in the band.”

Years of raucous St. Patrick’s Day parties, independent album releases and steady touring — like tonight’s gig at Diamond Ballroom — have afforded the act a loyal following and some unexpected fans, like the family of Woody Guthrie, who invited the band to tour his archives and encouraged them to bring some of Guthrie’s old, unused lyrics to life. The song became “I’m Shipping Up to Boston,” and rocketed the Murphys to new heights.

“There were just pages and pages of lyrics that no one understood what they were intended for. It had pretty much been relegated to a file cabinet,” Casey said. “He spoke his mind. He was so fearless … a real spirit of rebellion.”

Their latest record, “Going Out in Style,” is a concept album, a narrative about a fictional Irish immigrant recalling his life from beyond the grave. But it’s still designed as a glorious sing-along.

“It’s like we make records so people can learn them and sing them along with us. That’s our payoff at the end of the road, playing the songs to a roomful of people who know the songs,” Casey said. “We feel like the audience is the eighth member of the Murphys. You can’t get that feeling anywhere else.”