SEAN MacBRIDE was 12 when his father was executed by the British for his part in the Easter Rising of 1916 and the poet Yeats came from Dublin to bring the details.

"Yeats taught me to fly a kite," MacBride said this week on a trip to Washington to receive an American University honor, "and he was a very good instructor."

When grief is especially sudden or especially shocking, it is important to fly a kite or row a boat or construct a grape arbor - some activity at once physical and creative and preferably unobserved.

His father was Maj. John MacBride, who had fallen into substantial disfavor in England since he fought on the Boer side of the Boer War and was interested in Irish freedom from the crown.

But it would all have been different - his son would surely never have been in Washington last week, among other things - if the elder MacBride had not accidently run into some fellows digging trenches and found a small revolution was afoot and grabbed a rifle. That was Easter Monday, 1916, in Dublin's St. Stephen's Green, and before it was over he had become one of a handful of Irish heroes whose names are carved in stone at national shrines, though he was set in a grave of lime to await the Judgment.

The boy Sean, and his mother, Maud Gonne, were living in Paris to evade arrest by the British. News was censored then, but Yeats made a trip over to give all the details. When they offered the major a blindfold, he said, "Nonsense." All his life he had been looking down gun barrels at the British, he said, and he didn't need anybody's blindfold.

Well that was long ago. The boy Sean, now 74, is old and looks older. The blue is pretty well bleached from his eyes, and he's not really up to a typical American University reception - though he made a speech to the graduating law class, and will certainly make a speech up to the last minute Abraham calls him to his bosom.

He is metaphorically crowned with his Nobel laurels (the 1974 peace prize for his work as a founder of Amnesty International) and is United Nations high commissioner for Namibia. He was a major figure in the Irish Republican Army for two decades, later a foreign affairs minister for Ireland, and a great figure in non-governmental peace organizations like the International Peace Bureau (Geneva) of which he is president.

He does not seem much interested in the pleasures of the table, though he ate the mound of crabmeat sent up to his room at the Mayflower. Sardines, turnips, oatmeal - anything would have done as well, one suspected.

"You sprang your mother from jail twice when you were a boy," I ventured, to get him going.

"How did you know that? Hardly anybody knows that," he said, with a devilish grin like a grandfather caught eating cake on a sugar-free diet.

Well, he said, it was simple enough. His mother returned to Ireland, or tried to, but was spotted in London and arrested. She was allowed to go out for a Turkish bath and the fuzz who accompanied her then dropped off for a beer. Watching this pattern, young Sean figured out her escaped and the two of them got to Dublin.

In very little time her passion for nationalist politics landed her in jail again in London.

This time she was sent out to a rural prison - a nursing home - and again young Sean observed a hiatus once a day when guards were not present. Once again he sprang her free to Ireland.

She herself was, as any Irishman of reasonable years could tell you, the most beautiful woman of Europe. She was an actress, but it was Yeat's passion for her - he had proposed unsuccessfully in 1899 and never quite got over it - that established her as the only great beauty of consequence between Venus and Greta Garbo.

"Was it hard growing up the son of two national idols?" someone asked. "When other kids were eating ice cream cones and kicking the can - "

"I never thought of it as difficult," he said. 'It all seemed very natural, what was expected."

As a son of princes gets used to deference, or the child of high-wire artists sees nothing odd in heights.

"Old Maud," as everyone called the impressive Maud Gonne later on, when she was in her 80s (she died in 1953), did not lack the common tiger streak of ordinary mothers when their little Seans are wrongly persecuted.

Whe Sean was 14 he was arrested (for demonstrating at a trial and for carrying a pistol butt he said a Frenchman gave him) and put in jail. His mother raised something of a racket - dear Lord, the boy was a mere tad, etc.

"I was furious," MacBride said. Here he was, a man among men in jail, where all honorable Irish men ought to be, bearing up like a man, when his mother starts hollering about his being a little boy.

The British released him, to his chagrin.

"I had been pretending I was 17," he said.

Well that was long ago. Before his father's execution even, he knew something of violence.

In the south of France, where they lived for a time, his mother worked with wounded soldiers in a World War I hospital, and Sean made himself useful lighting the cigarettes of soldiers who couldn't light their own, missing arms.

Today MacBride is regarded by some as a sentimental utopian. Now that he is too old to throw bombs (the implication runs) he has settled in as a prophet for peace, without having the foggiest notion of the dangers he espouses, chiefly disarmament.

"It's amazing," he said, with a reproachful look at the crabmeat, 'how few people remember that in 1961 both the Soviets and the Americans were agreed on the principle of complete disarmament - not just an easing of arms production, but the abolishing of arms in excess of those needed for domestic security."

Since then, of course, there has been Vietnam.

The trouble is (he never tires of pointing out) that when it comes to war the people are not consulted at all. They may be propagandized, manipulated or otherwise prepared, but they are not consulted, and only rarely are they able (as in the case of Vietnam) to reverse the government decision for war.

Presidents invariably consult, he said the military chiefs of staff - sincere God-fearing men, MacBride says he does not doubt - but men of special training in the proposition that wars will come and they must fight them and must therefore be prepared with the best arsenal possible. Such men, MacBride adds, traditionally are not especially intelligent, let alone wise, but their counsels are likely to prevail with presidents, chiefs of state.

The chief of state knows little of military matters, and is afraid to turn a deaf ear to the dire warnings of the military establishment.

In addition, MacBride argues, a Western society commonly has tremendous banking and corporate powers who do quite well in an economy ginding for war in its arms production.

Arming everybody and his brother in the Middle East inspires a particularly sharp curl of his lip, and he has a special bitter laugh for such precautions as making people who buy weapons promise not to use them in any naughty way.

He reads, attends international conferences, makes speeches, drums drums. There is a bit of the populist in him, and he could hardly be expected to resist the opportunity (addressing the law graduates) of saying President Carter "rightly criticized the selfish and unsocial attitude of lawyers," since much of the criticism is "richly deserved."

Some Westerners have been put off by MacBride's view that socialist countries (Russia) can adjust their economy to arms control easier than the westerns (America) powers, and it is easy to infer that he means the arms race continues because America, among others, finds it profitable to continue it.

He has great faith in th forthcoming special United Nations session (May 23) on disarmament, and he does not seem much shaken if he is called a dupe of Soviet rhetoric.

His reasoning is simple enough: Weapons will be used, if they exist. Therefore they must not exist. And it is his job, as he sees it, to rouse support for a direct confrontation of the question of what the hell the bombs are for, and who really thinks the world will be livable after a nuclear war?

He sees the world in a perhaps temporary state of moral collapse, and has fussed with energy at churches for not providing much of a lead.

"Keep under strict surveillance and control," he said to the young lawyers, "those secret establishments which within your government structures seem to regard themselves as above the law."

And "lawyers should never hesitate to expose the covert operations of secret government agencies or the corrupt practices of wealthy multinational corporations."

He was given the new American Medal of Justice by American University for contributions to justice.

He needed a violet velvet hood for university ceremonies and the fellow who rents them seemed a bit suspicious, but agreed to charge the hood rental to the dean of the law school. Ceremonies ran overtime and the reception was 45 minutes late in starting.The few early birds got lots of sandwiches.