On Easter Sunday, while on her afternoon stroll, the Irish novelist Denise Deegan realized she still had not yet called her mother. “Hello,” she said cheerily into her phone. “Hello,” a man on the street replied.

Looking at the man’s face, she realized the voice belonged to the actor Matt Damon.

Living through a pandemic already had her feeling as if she were in a movie, “and then you look up and Matt Damon’s there,” she said in an interview.

Sightings of Mr. Damon have become common in recent weeks in Dalkey, a seaside resort town southeast of Dublin, where his presence has added yet another surreal layer to life under lockdown.

It’s not just that one of the world’s biggest stars and his family must stay within two kilometers (about 1.2 miles) of home, like the rest of them. It’s that an actor who played a father trying to protect his family amid a sprawling pandemic in the movie “Contagion” is now living through an eerily similar reality alongside them.