I've known Kiera Duffy for years. She was this wiry teenager, hunched over her adoring grandfather and singing along with the rest of us as he played the melodies of “Oklahoma” during their family parties.

I ran into her every now and then behind the cash registers in the stores around Nassau Street in Princeton as she worked her way through school at the nearby Westminster Choir College.

Could this cheery thing now be Kiera Duffy, the emerging opera star who opened the 2008-‘09 season with her New York Philharmonic debut in Pierre Boulez’s “Pliselon pli:Improvisation II sur Mallarmé” under the baton of music director Lorin Maazel?

Is this the same sensational soprano who goes on to sing György Ligeti’s seminal works, “Aventures” and “Nouvelles Aventures,” for her first performances with the Los Angeles Philharmonic?

After making a name for herself on this side of the Atlantic, is this same girl behind the cash register leaving for Ireland this week to join the Opera Theater of St. Louis for a performance of “Ghosts of Versailles” at the Wexford Opera Festival?

That same Kiera Duffy is a mixture of schoolgirl and opera star. In our interview, she sounds like a mall rat when she squeals, “Omigod, that is such a good question” before revealing herself in a highly intelligent, introspective response to my inquiries.

She speaks about the joy of mastering Bach on the piano at eight before reducing herself once again to a schoolgirl in professing her obsession with Radiohead.

“I’m in a little bit of denial that I am going to Ireland because I have so much to do, but I am really getting jazzed this week,” she says.

“I was doing a workshop for a new opera in New York last spring and it was there that I met the Wexford Festival director, David Agler. He and I actually went to the same college.

“He said he had a role in mind for me, and while Ireland is not the first place you think about when you think of opera festivals in the way that you think about places like Germany and Italy, I had known about this festival for a number of years and was dying to play it. Wexford has this brand new 800 seat opera house and the acoustics are amazing, so I have high expectations for the show!”

Born in Philadelphia, Duffy was an accomplished pianist before pursuing singing and holds bachelor’s and master’s degrees in voice performance and pedagogy from Westminster Choir College.

“I discovered opera serendipitously,” she says. “I went to college to be a choral conductor because I wanted to use my skills as a piano player with the vocal experience I was getting toward the end of high school.

“I had no aspirations of being a solo singer. When I went to college, you couldn’t be a choral person in your undergraduate work, and my high school teacher who was also a graduate of my college encouraged me to try for the vocal department.

“I did an opera role in the junior year in college. I developed this high range and this voice inside of me revealed itself. My teacher, Laura Brooks Rice, continued to encourage me to go for it, and 12 years later she still teaches me today.”

Of course, few of us move beyond the many a star turn by the piano for intimate family gatherings, Duffy she found a way.

“We were always a musical family on both sides, but I don’t have to tell you that,” she jokes. “We were obsessed with “The Sound of Music,” “Oklahoma,” “Carousel,” “South Pacific,” and “Camelot” -- all of these great musical theater scores to the shows from the fifties.

“That was my introduction to vocal music. I had studied classical music on the piano for 14 years, since I was four. My aunts Debbie and Diane were classically trained as well.”

Like any starving artist, she moved to New York, did temp work in financial firms and hated every minute of it. She was committed, in her words, to “hold fast, pray for a lucky break, and work hard.”

Most sopranos in her field are the circumference of a small moon orbiting Mars, which makes the voice that springs from Duffy’s small frame all the more remarkable.

“To the outside voice I sound loud and I am amazed that the voice comes out of me, though in opera circles my voice is on the small side,” she explains.

“I don’t think I was a natural singer but I had a great teacher who taught me about breath and resonance, optimizing your anatomy to make this internal amplifier. It allows a small person like me to make this sound. 

“Bigger bodies have bigger rib cages and that gives you a bigger power source. The air you take in is like the gas in the engine.”

Duffy is an engine that is revving up for the trip across the Atlantic for her gig in Ireland before returning to the States in the winter. To follow this rising star, log onto www.kieraduffy.com.