In March, one of the busiest men in Washington is the new Irish Ambassador, Sean Donlon.

"There's a bakery in New Haven that wants to present me a giant green bagel," the diplomat mused, relaxing briefly with a cup of coffee in his office this week.

"I have a shamrock being flown in fresh from Ireland to present to President Carter, and I can't remember how many radio and television programs want me to come around for a chat. In March, there are all kinds of audiences I can reach that would be hard to reach at other times."

For the whole month, in fact, Donlon says he has scheduled "only one thing that is not Irish -- the chili cookoff at the Congressional Club. Since I have never tasted chili in my life, I thought I would be an excellent judge, completely without prejudice."

In the six months since he began the job, the 38-year-old ambassador has found the most time-consuming role is the unofficial one: ambassador to the 20 million Americans who claim Irish ancestry.

"We're part of the three I's, you know," Donlon says. "I frequently compare notes with my Italian and Israeli colleagues and find that we have many activities in common. Being ambassador to the United States is very much a Washington job, and the other is very much an out-of-Washington job."

Washington is Donlon's first post as a full ambassador, his second assignment in the United States. (He is also the Irish ambassador to Mexico.) He was consul general in Boston (only slightly less Irish than Dublin) from 1968 to 1971, and recalls it as "the easiest job I have ever had.

"There was no problem an Irish consul couldn't solve in Boston -- everywhere you went, you found friends of Ireland, and at night you could wander into a pub in Cambridge and solve all the world's problems in an hour."

Getting Washington as his first appointment means, Donlon says that "it will be all downhill from here, unless Massachusetts declares its independence and I can get myself appointed ambassador there."

Donlon is considered young for an ambassador, but "in Ireland I'm an old man. Half the people in Ireland now are under 25 years of age."

Donlon says he entered diplomacy "almost by accident, the way everyone does, I guess. I began as a student for the priesthood at our national seminary, Maynooth, and then I decided that the clerical life was not for me.

"It wasn't the vows of poverty or obedience that stopped me. It was that other one -- chastity. So I switched to the study of history.

"I began working for the gobernment in the Department of Finance. I went into diplomacy in 1963 and spent five years in Bonn because of my interest in politics. Diplomacy is as close to politics as you can get withour actually taking a public office."

It is often close to business administration, too. Promoting foreign investment and increasing the American tourist trade (300,000 last year) are a large part of Donlon's assignment. With the vast emigrations of the past now over, and the population decline (from 8 million in 1850 to 4 million in 1950) reversed, Ireland is in a period of strong potential growth, he says.

"There is a great friendship for Ireland in this country," he says with a genial smile, "and we will shamelessly capitalize on it. We must create about 30,000 new jobs each year for our young people, and half of that has to be done with foreign investment."

"I think the troubles in Ireland have brought a new awareness here," Donlon reflects, "a new interest in the facts to go along with the perennial interest in the folklore.

"When someone asked me recently why there is so much good will for Ireland, I said I couldn't explain it, except that we are the most lovable, intelligent, articulate, numerant and humble people in the world."

Numerant?

"That means we're very good at counting -- especially money."

The Irish counted for a great deal in the 1840s. When a blight on the po-American population was Irish-born tato crop caused famine and mass emigration. "By 1871, 48 percent of the -- not of Irish ancestry, mind you, but natives of Irland," Donlon says.

That influence remains today. "We did a study" Donlon says, "and we found that 25 percent of the members of Congress have some Irish connection -- and if you add people who married women with Irish connections, it goes much higher."

"In another generation," Donlon says, "We hope to complete our colonization of America."

Meanwhile, Donlon has more than enough to do. "I may begin a typical morning discussing beef exports, then tax changes and then shipping regulations with various people -- then at lunch the subject may be oil technology, or I may e trying to persuade a large computer company that they want to set up a sub-assembly plant in Ireland.

"And there are my regular jobs of reporting back to Ireland and running an office -- all before my first cocktail party at 5 p.m., which is supposed to be when an ambassador really gets to work."

Donlon also finds that a large part of his job is persuading well-meaning people that they are not helping Ireland by giving aid to terrorists.

"Our goal is first to make Ireland a better place for Irishmen to live in, and second to resolve the political problem resulting from our two traditions -- one Catholic and one Protestant.

"There is a common Irishness and there is a growing conviction that the problem will be solved by Irishmen in Ireland -- once England, which created the problem, has worked with us to create a situation in which the British can leave."

On a happier note, Donlon observes that some reverse immigration has begun. Foreign writers, artists and musicians are moving to Ireland because of a law, passed in the 1960s, that gives a tax exemption to income derived from creative activity.

"Besides attracting many foreign artists, it has promoted a renaissance in Irish art," he said. "We're great people in Ireland for recognizing artists after they're dead, but now we're helping them while they're still alive.

"We have many new poets and play-wrights coming along in the last 10 or 15 years, and painters and sculptors as well. There are no great composers yet, though three or four people will shoot me for saying that -- some of them are very good, but nne has yet made an international reputation."

Donlon believes the musical renaissance is bound to come because Ireland has a great folk music tradition and some internationally famous performing artists -- flutist James Galway, for example.

Launched on a favorite subject, the ambassador modestly neglects to mention his own musical reputation among those who have heard him sing and play piano at Washington parties.

Of course, all the Irish are born great tenors," he says, "except the ones who are born great sopranos."