Roy Cohn stares into space when he talks. He watches his questioner neither as friend nor accuser, but only as an object. His full-orb eyes seem always to have their shades half drawn as he talks on. He was talking Saturday to honor the memory of Joseph McCarthy. About 100 people gathered at St. Patrick's Cathedral for a mass for the former senator 25 years after his death, and later listened to his favorite acolyte, Roy Cohn, the enfant terrible who is enfant no more.

A young woman asked Cohn, "Ultimately do you think, I mean what do you think, about Joseph McCarthy's place in history? I mean do you think it will be good or bad?"

Cohn looked to the other side of the banquet room at the Warwick Hotel and spoke to nobody in particular. "He was a person of good intelligence and instincts, who was able to see what was going on in the world." This satisfied the woman, and Cohn went into a litany of the reasons. McCarthy's redemption would be imminent, he said, chiefly through the politics of Ronald Reagan and the repudiation by the American people of such "late unlamented senators" as George McGovern, Frank Church, Birch Bayh and John Culver.

Twenty-eight years ago Cohn's face was that of a baby and all of America watched him live on television in the Army-McCarthy hearings. Within a year of his apex of fame McCarthy was censured by theU.S. Senate and on May 2, 1957, flattened by the establishment he had attacked, his memory no longer threatening,, he died at Bethesda Naval Hospital.

Most of the 100, the last faithful of Joseph R. McCarthy, were thin and gray and in their sixties or older. They had just attended, as they have almost every year since McCarthy's death, the mass at St. Patrick's. They talked in low voices to each other and one man said he had just spoken to Kate Smith on the phone and even though her diabetes had cut her weight down to 100 pounds, she was in healthy shape and ready to go on with life. Waiters brought around fruit salad cups.

A conservative theorist named Dr. Timothy Mitchell stood at the dais and said that Roy Cohn needed no introduction. Cohn looked neither at him nor anybody else. He didn't touch his fruit salad. Constance Christopher, a short woman with henna-colored hair, sang "The Star-Spangled Banner" in fine voice a cappella into the microphone. A heavyset woman with a great deal of raven hair piled high on her head sat on Christopher's right and sang louder and more operatically, beating Christopher on the high notes by an entire octave. The man who had spoken to Kate Smith smiled and said he wished he had her here. Cohn stared off to his right, speaking to nobody.

But when he was introduced life flushed into his face and he jumped up and told a joke which he said that he had heard Reagan tell. The mention of the president's name got a great deal of applause and one old woman yelled "Hear, hear!"

"Any 25th anniversary," Cohn said, "is a matter of great significance. Due to the thoroughly inadequate work of historians and teachers, McCarthyism is still not understood." Cohn said that thanks to their intellectual curiosity, though, young people are beginning to understand McCarthyism as it was and this had led to the election of Reagan and Republicans around the country and the repudiation, once more, of the "late, unlamented" McGovern, Church, Culver and Bayh.

"Sen. McCarthy stood for one thing only: An early warning system that was launched off one ism, Nazism, and based on the threat of a new ism, communism," Cohn said.

"It is essential,"Cohn said, looking at nobody in particular, going on without real heat in his words but without faltering even for a millisecond, "that the United States be armed and strong. If there's one person present today who believes the reason we want to be strong is not to prevent war but to cause war, that person is not worthy of the name American."

The celebrants banged on their tables. "Hear, hear!" many of them said. Cohn said the media is intent on bringing down Reagan as they had brought down Richard Nixon. "Hear, hear!" many called out. "We face challenges," Cohn said. "We have some very important elections coming up. In Connecticut Lowell Weicker is running for reelection." "Boo--boooooo!" the celebrants yelled.

"And in New Jersey, we have a female--and it's nice to have a female who can be as threatening as a man can--that is this nice, lovely, grandmotherly lady named Millicent Fenwick whose voting record is higher on the Carter administration's approval rating than George McGovern. I think we have to do something about that." The celebrants banged silverware.

Cohn then got to the clarion call of the 25th anniversary observation of the death of McCarthy. He said he would like to see a "return to the strong congressional committee that would bring us to a point where we can investigate internal subversion in this country.

"A few things I would like to go into with this committee are: Why is the United States still in the U.N.? That's a particularly appropriate question for a congressional committee to study." There was a great deal of banging of silverware. "I remember being with Sen. McCarthy," Cohn said, "and hearing a story when somebody asked 'Is there a place for a Red Chinese seat in the U.N.?' 'Yes,' the senator answered, 'Ours.'

"And I'm sure that if you asked our great ambassador, Jeane Kirkpatrick, that same question, I dare say you can get the same answer. There's an anti-American bloc in the United Nations and I'd like to know why we are trying to give taxpayers' money into the hands of those who do everything they can to destroy everything for which we stand."

Cohn then looked out at his audience for a coup de gra ce. "And another appropriate question, I believe, has to do with studying people in Hollywood. I would like to know why these people think they can take the money of ticket buyers and put them into communist coffers. Just as in the old days when we had an investigation into the participation of communists in Hollywood I'd like to know where people like Ed Asner are coming from these days."

The silverware almost jumped off the table as fists went up and down on the cloth.