The night manager of a popular Rockville restaurant was fatally wounded Tuesday night when two armed robbers forced their way into the restaurant and surprised him as he was counting receipts.

According to Montgomery County police, the gunmen, described as about six feet tall and wearing stockings over their heads, entered O'Brien's Pit Barbecue through the kitchen at 10:15 p.m., 15 minutes after the restaurant's closing. After herding five employes into a small office, one of the gunmen entered the dining area, where Michael O'Callahan, the 45-year-old night manager, was going over receipts.

A single shot was heard almost immediately, police said. After rifling the cash box, the gunman rejoined his partner and took an undetermined amount of money from the office safe before escaping through the side door.

A rescue unit rushed O'Callahan to Shady Grove Adventist Hospital, but he was dead on arrival from a single wound in the head, according to police. It was the first murder in Rockville this year.

Jerry Schrader, a part owner of O'Brien's, said yesterday he thinks the assailants were "very familiar with the routine" of the restaurant's closings. He said O'Callahan, who lived in Montgomery Village, was one of the original people who helped open the restaurant on Gude Drive 12 years ago, and that there are plans to set up some sort of trust fund for O'Callahan's two children.

O'Callahan was born in County Cork in Ireland and came to this country in 1964, according to his wife, Bridget. "He was one of a kind," she said yesterday. "I'm angry and upset, but I would feel the same way if he had died of cancer. I'm not angry with anyone. I'm just angry it happened.

"How else do you feel when someone you love has been taken from you?" she said.

For the merchants who occupy the small shopping center where O'Brien's is located, the violence that occurred in their midst was a bad shock.

"It makes you a little nervous," said Ron Deck of the nearby Wright's Furniture store, adding that his fellow shopowners were unaccustomed to violent crime. "It puts you on edge. We've never had any trouble like this."

Another merchant, Fred Harris, said the restaurant is "pretty isolated at night" because the shopping center is located in an industrial area near the Gude Landfill and is the only business that remains open late.

Friends and acquaintances speculated yesterday how O'Callahan reacted when he was surprised by the gunman. Some said he clearly wouldn't have challenged an armed man, especially since one of his legs was in a cast from a soccer injury. But others, like Harris, described O'Callahan, a tall, solidly built man who still spoke with an Irish brogue, as the sort who "wouldn't take any crap off anybody."

According to police, the five employes heard no exchange of words between the robber and O'Callahan. "They said they just heard one gunshot. There was no sign of a confrontation," said Phil Caswell, a police spokesman.

Authorities said yesterday that they have no suspects and the murder weapon has not been recovered.