"We've never had a rainy parade yet," said Martin J. Hannon, president of the 1,000 menber Irish-American Club of metropollitan Washington. "St. Patrick really came through today."

In fact, he came through just in time. A morning downpour on Washington stopped just three hours before the beginning of Washington's sixth annual St. Patrick's Day Parade, which drew a record 120,000 spectators, according to police, along Constitution Avenue yesterday afternoon.

Then, as quickly as they had cleared, the skies grew dark again and rain resumed by 5:30 p.m. with weathermen predicing thunderstorms for the evening.

As temperature inched within two degrees of the day's record high of 74 in 1890 an magnolias blossomed on the Ellipse, Irish dancers, a Chinese dragon, a file and drum crops rom Virginia and high school bands from the District marched in the two-hour parade, which only six years ago had drawn an audience of only 200 people .

Hannon sais ht parade - billed this year as "A little Bit of Heaven in "77" - was only 15 minutes long when it first was held. "We're getting better everytime," he said.

But the tense events in the city last weeK, when gunmen held 124 hostages in three different locations for 40 hours, had put the parade in jeopardy according to parade committee chairman Matthew J. Hannon. "We were in flux for awhile. I was afraid we might not be able to get the police here if it was still going on," he explained.

Hannon sai a few units had cancelled when they heard of the hostage situation, but then had "reinstated" themselves.

"This is a fun occasion," Hannon said. "It is nonpolitical and nonreligious and we tried to make it as ecumenical aspossible," he added, pointing out the presence of Arlington's Bishopo Thomas J. Welsh and Washington National Cathedral's Canon Michael Hamilton who was born in Ireland, according to Hannon.

"We tired to get an Irish born rabbi, as we have had in the past, but we just weren't able to this year," Hannon said.

Among the units marching in yesterdat's parade, which was financed by donations from the 30-year-old Irish-American Club, were Ancient Orders of Hibernians and their auxiliaries from Arlington and the District, and the Pottsville Pa. Area High School.

A truckload of jovial celebrators from a local pub, some drinking an unidentified green beverage in Skippy peanut butter jars, cheered Washington's mayor, Walter E. Washington, as they passed the reviewing stand.

Mayor, Washington, with a wilted shamrock in his buttonhole, sat next to the Irish Ambassador to the U.S., John G. Molloy. Did he have a little bit of Irish in him? "I like to think I do," said the mayor.

On the other side of Molloy, in a dapper green velour derby and with a shillelagh restingon his knee, sat AFL-CIO President George Meany, who gave someone other than St. Patrick credit for the beautiful weather. "The mayor arranged some good weather for us," he joked.

District Police chief Maurice J. Cullinana led the parade to loud applause from the sidelines. Asked about his enthusiastic response. Cullinane, whose grandparents came from Ireland said," Well, this is my constitutuency."

Among the high-spirited spectators was Deputy Police Chief Robert L. Rabe, who pronounced himself "very happy" now that the hostage event was over. Rabe had participated in the tense three-hour face-to-face negotiations between the gunmen's leader and three foreign ambassadors.

And when the Bavarian dancers passed the reviewing stand Rabe turned to Washington and Cullinane and quipped, "You're in trouble now, here come my people."

Also in the crowd were 7-year-old Beth Ann and 5-year-old Dierdre Brady wearing "Kiss me, I'm Irish" pins. Their father, John Brady of Silver Spring said he had brought all his children - "at last count, eight" - to watch the parade. "And if there'd been nine, I would havebrought all of them too," he added.