Sitting in her dressing room on Tuesday at Irish Repertory Theater in Manhattan, talking to me about her latest role, the actress Kate Mulgrew initially sounded like herself: an American from Iowa who happens to share a voice with Kathryn Janeway, the Starfleet captain she played on “Star Trek: Voyager.”

A minute or two into the interview, though, a Dublin accent started shading some of her phrases, and soon it was coloring all of them. That’s the first thing you need to know, because when you read her words here it helps to imagine their cadence as they hit the air.

The second thing to know is why she would slip into that lilt and sustain it for nearly an hour. She was simply holding tight to Beiv Scanlon, the character she is playing in Nancy Harris’s thriller “The Beacon,” on the Irish Rep main stage.

Not that Mulgrew, 69, has been speaking with that accent constantly, but she has been doing it “a lot,” she said. “Yesterday I didn’t. I had to go off and do some things, and I didn’t want to disconcert people who’ve known me for years. Right? That would be odd.”

But if, offstage, the accent can be discombobulating even for those of us who don’t know her personally, it’s all in service of Beiv (rhymes with wave).