No one has ever accused Marie Gallagher of being shy. Still, the 78-year-old grandmother from Revere never expected her gregarious personality would lead to a starring role in a national Volkswagen commercial.

“I’d never done any acting, although I do love to sing,’’ says Gallagher. “And I didn’t really know what they expected. I just thought it might be fun.’’

The ad for Volkswagen’s Atlas SUV features Gallagher as a woman fulfilling her late husband’s wish to take her multi-generational clan on a road trip across the country, set to Simon and Garfunkel’s “America.’’ The mother of four, a longtime widow herself, provides the voice-over and serves as the commercial’s anchor despite her lack of acting experience. The ad includes a shot of her looking at a photograph of a young couple; in fact, it’s a picture Gallagher and her late husband, John.

“I brought in some pictures, including ones where we had everything we owned strapped to the roof of the Volkswagen we owned, including our son’s crib,’’ she says. “They ended up using one of the photos of my husband, John, and me in the commercial. That made it easy for me to believe in what I was doing.’’

Casting a novice in a high-profile car commercial seems unusual, but Ashley Skomurski, a partner in Boston-based Slate Casting, says “real people’’ casting has become a trend lately.

“Directors are often looking for someone who can play a role without affectation,’’ Skomurski says. “In this case, the director was looking for an Irish grandmother in her 70s who could give an authentic performance.’’

Skomurski put up fliers in various places, including the Irish Cultural Centre in Canton, where Gallagher’s friend Shawn Doherty, an actor himself, saw it.

“He told me I was perfect for it,’’ Gallagher says. “And I thought, ‘Why not?’ ’’

Gallagher still has a thick Irish brogue even after living in the United States for nearly 60 years. She may not have had any acting experience, but she has always been open to adventure: When her children were young, she traveled across the country while her husband worked as a lineman installing power lines in Mississippi, the Midwest, and the Southwest.

And she’s no stranger to performing: She sang country-western in a band in upstate New York and Irish tunes for choreographer and former Bill T. Jones dancer Sean Curran when his company performed around Boston. She also hosted a cable TV show in Boston for six years that featured videos of Irish music performances. Her work included a 15-year stint in the Harvard athletic office as well as a part-time job at a construction company that she just left last month.

While casting the ad, Skomurski met with about a dozen candidates in their homes. “By interviewing people at home first, we can see how natural they can be in front of the camera,’’ she says.

After passing that initial audition, Gallagher had to endure auditions in Boston and then New York before she was hired.

“I was the last one they talked to in New York,’’ Gallagher says. “After seeing everyone else coming in and out, I had to check to make sure my name was still on the list.’’

During her audition, Gallagher was paired with various combinations of other actors who could play her adult children and grandchildren. She says she wasn’t given a script to read, just told to do what she felt like doing to keep her fictional family happy, since they’d recently lost their grandfather.

“So I said, ‘Let’s sing a song for Grandpa,’ ’’ Gallagher says. “That threw them for a loop, but it made it fun for everyone.’’

Skomurski says Gallagher did the same thing in her home when she had interviewed her.

“Marie is so easygoing and able to ignore the camera,’’ she says, “but singing really made her comfortable.’’

After completing the ad, which involved a month of shooting at locations around the country, Gallagher wasn’t done with acting. She appeared as an extra in the Will Ferrell film “Daddy’s Home 2,’’ which was also cast by Slate Casting and filmed in Boston.

“We were in Logan Airport for three 13-hour days,’’ she says. “I stood it better than some of the young ones.’’

She has joined the Screen Actors Guild but says she’ll have to get an agent if she really wants to do more.

“We’ll see what comes next,’’ Gallagher says. “I’m enjoying this right now.’’

In the meantime, you might catch her at The Burren in Somerville, where she occasionally steps up to sing an Irish tune.

“I don’t have the range I once did,’’ she says. “But I can still draw a tear from a stone.’’

Terry Byrne can be reached at trbyrne@aol.com.