PORTLAND — The Dropkick Murphys, one of the nation’s top punk rock bands, announced this week they will join Mumford & Sons during the highly anticipated concert festival on the Eastern Promenade on Aug. 4.

The Massachusetts natives — perhaps best known for the punk rendition of “I’m Shipping Up to Boston” used heavily in the Oscar-winning 2006 movie “The Departed” and in Boston sports arenas — represent a surprise second headline-worthy act in the festival.

“We hung out with our buddies Mumford & Sons a few weeks ago at a festival in Europe, and they called [Tuesday] to invite us to play their Gentlemen of the Road Festival up in Portland, Maine, on Saturday, Aug. 4,” Dropkick Murphys bassist and singer Ken Casey said in a post on the band’s website. “It sounds like a great time, so we’re going to jump out of the studio to close the second stage.”

The Dropkick Murphys enjoyed regional success and a loyal following for many years before the aforementioned song delivered the band more mainstream exposure. The group’s most recent two albums have reached the top 20 on the Billboard charts, with the 2011 record “Going Out In Style” peaking as high as No. 6.

The Dropkick Murphys’ late addition to the Aug. 4 festival lineup promises to pump even more star power into the event, but it won’t make it any easier to get in. Tickets have gone fast, with fans eager to see the British folk rock band Mumford & Sons, who picked Portland as one of only four U.S. sites to include in their Gentlemen of the Road tour.

The initial wave of tickets for the festival stop sold out in less than an hour on June 1, and a second batch of 3,000 was released two weeks ago. But Casey promised a backup plan for fans who couldn’t get their hands on ducats in time.

“The festival is sold out, so we’re hoping some of you already have tickets,” he wrote. “If you don’t have tickets, don’t worry! As part of the Gentlemen of the Road Festival, all the bands will be doing their own late night shows throughout Portland. We’re lucky enough to be doing our own show at The State Theatre. The opening band will be our friends from Cape Cod, the Parkington Sisters.”

The seven-hour Mumford & Sons festival — featuring two stages and several bands leading into the headliners — is scheduled to wrap up at 9 p.m.

The Parkington Sisters will take the stage across the city at The State Theatre at 10:30 p.m., Casey wrote, with the Dropkick Murphys to follow at 11:30 p.m.

There were 15,000 tickets sold for the festival date overall, with those ticket holders reportedly getting first dibs on the late-night spin-off shows as well. Remaining tickets for the Dropkick Murphys’ 11:30 p.m. performance are scheduled to go on sale to the general public at 10 a.m. Friday.

Gates open to the Gentlemen of the Road festival site on the Eastern Promenade at noon on the day of the show, and Munjoy Hill will be closed to all but residential traffic the entire day.

Out of each ticket sale, 50 cents will benefit the organization Friends of the Eastern Promenade, and Mumford & Sons band members plan to ask homeowners in the area of the stage to decorate their homes in an informal contest. The best-decorated place may receive a personal visit by the band.

Other performers slated to join Mumford & Sons and the Dropkick Murphys in the festival lineup include The Maccabees, Apache Relay and Simon Felice, among others.

Setup for the festival will begin on the Thursday morning before the Saturday concert, with two Cutter Street parking lots closing down at 7 a.m. and stage assembly beginning at 8 a.m. on Aug. 2. On Friday, Aug. 3, after Mumford & Sons is scheduled to hold a special in-store performance at the Scarborough Bull Moose music store, the band’s sound check on the Eastern Prom will take place from 6 to 8 p.m.