When he was 17 years old, in 1984, Brian F. O’Byrne had seen only three movies in his entire life. That 20 years later he would fly to Los Angeles to appear in one with Clint Eastwood almost defies his imagination.

In “Million Dollar Baby,” Mr. O’Byrne plays Father Horvak, confessor to Mr. Eastwood’s character, Frankie Dunn. And while his four scenes may squeak by without particular notice, given the bright Hollywood lights on Mr. Eastwood, Morgan Freeman, and Hilary Swank, on Broadway, the spotlight can’t stop following the 37-year-old Irishman, currently co-starring in John Patrick Shanley’s new work “Doubt,” in which he plays – again – a priest.

Reaching toward a metal bar suspended from the ceiling in his dressing room at Manhattan Theatre Club’s City Center Stage I, Mr. O’Byrne tugged the green vestment hanging beside the cassock he wears as Father Flynn, quipping, “I’ve been wearing this costume all year. I’m the man to go to with any spiritual acting need,” said the County Cavan, Ireland-native. “I’m hoping to move on,” he added with mischievous glimmer in his eye.

What is it like to work with Mr. Eastwood? “It wasn’t like being on a movie set,” Mr. O’Byrne said. “It didn’t even feel like there was any acting going on.”

There is plenty of acting going on elsewhere for the performer. Last year he won a Tony, Obie, Drama League, and Outer Critics Circle Award for his riveting performance as a murderous pedophile in Bryony Lavery’s “Frozen.” Asked about playing the role of a villain, he impishly replied, “I love it, because I’m a coward. I’d run away if somebody said ‘boo’ to me.”

“My favorite thing is new work,” he continued. His Broadway debut, in 1994, was a small part in Wendy Wasserstein’s “The Sisters Rosensweig,” followed by major ones in Martin McDonagh’s “Connemara Trilogy,” which Mr. O’Byrne also performed in Ireland and London. In 1998 and 1999 he earned back-to-back Tony nominations for “The Beauty Queen of Leenane” and “The Lonesome West.”

In addition to good material, he has been blessed with fine co-stars. “They’d make anyone look good,” he said of Swoozie Kurtz, who co-starred in “Frozen,” and Cherry Jones, with whom he shares the stage in “Doubt,” adding, “You’re only as good as the person playing opposite you.”

In “Doubt,” Cherry Jones takes on the role of Sister Aloysius. Mr. O’Byrne said, “She’s a battle-ax – immovable.” Out of character and in street clothes, she is transformed. “I don’t recognize her,” he confessed, “She’s so beautiful, so soft.” Equally smitten with the talent of Swoozie Kurtz, he said, “I still think about Swoozie in that play. She just broke my heart.”

Doug Hughes directed Mr. O’Byrne in both dramas. “I’m really thankful that Doug let me do this,” the actor said. “I hadn’t done an American play before.” Believing that the director took a risk in casting him, he said, “I don’t audition well. I never know what I’m doing. The rehearsal process is a process of doing things wrong.” Once he gets it right, he said, it’s because “I’ve tried every other way to do it wrong.”

In Mr. O’Byrne’s subterranean dressing room at City Center, a cot neatly made up with sheets and a wool blanket took up half the room. One of the props, a basketball, sat on the linoleum floor near a few gym lockers, and Ireland’s tricolor and a County Cavan Gaelic football jersey cascaded from a hook near the bathroom.

Courtesy of Mr. O’Byrne, several production members with Irish roots now have similar jerseys. The playwright, Mr. Shanley, who won an Oscar for his screenplay “Moonstruck,” got a West Meath one; Mr. Hughes, the director, Langford; Elizabeth Moloney, the stage manager, Roscommon, and Stephen Gabis, the dialect coach, Derry.

The oldest of three children, born to a homemaker and the principal of his school, Mr. O’Byrne explained his early school years in Ireland. “It’s not fun to go home with the headmaster – particularly when he’s been administering discipline, and you’ve been the point of it.” His hometown, Mullagh, was “nowhere – 350 people and nine bars,” but 34 cousins provided a large extended family.

The future actor had no theatrical ambition during childhood. “I didn’t give myself many options, because I wasn’t a good student,” he said. Still, seeds got sown. “I was brought up around community theater- singing and dancing. It was something to do during winter nights.”

At his high school, which he described as “pretty uneventful” and “very uninspiring,” there were “no arts – not even one art class.” At Trinity College in Dublin, he discovered cinema through two friends and followed them to study at the Samuel Beckett Center. He auditioned and realized, “this is something I could do for the rest of my life.”

His teacher perceived a glut of actors and discouraged his students: Mr. O’Byrne remembered thinking, “I’m obviously not good enough.” So, after graduation, he “went to London to be with a girl.”

But fate intervened. Unknown to him, his uncle, an electrician in Queens, had entered his name, along with those of his younger sister and brother, into the annual lottery for green cards. Mr. O’Byrne had worked in construction in New York for a year between high school and college. All three siblings won them, and within two weeks of his 1990 arrival in New York, he got an acting job at the Irish Repertory Theatre.

In the opening lines of “Doubt,” Mr. O’Byrne intones as Father Flynn, “What do you do when you’re not sure?”

The actor expressed no uncertainty about whether Father Flynn “interfered” with a young black student at St. Nicholas Church School. “I know,” he said. “I couldn’t play the part otherwise.”

“All the way through the play, Shanley plays with the audience about whether the character’s guilty or not,” he explained. “At the final point of the play, the plot turns. He points the finger at the audience and leaves the audience with doubt.”

The actor wasn’t about to give away any more details. Folding his arms, he stretched his legs, crossed them at the ankles, and regarded his visitor slyly, momentarily slipping into character.

At the actor’s suggestion, his fellow players were not present when the playwright gave additional background on the fictional Father Flynn. “It’s driving them nuts,” said Mr. O’Byrne. “I’ve sworn to let them know after the last curtain call,” he said, but suggested that he may string them along for a while because the show will move to bigger quarters on Broadway. “I may play with them a bit,” he grinned. “Can we live with doubt in our lives?” is what the drama is about, he teased.