LAST MONDAY, after he had heard the disquieting news that Ronald Reagan couldn't decide between England and Argentina, the British Ambassador was gratified -- and surprised -- to hear different words from the speaker of the House, Thomas P. O'Neill Jr.

"Despite my breeding," O'Neill said, "I am on Mrs. Thatcher's side."

So am I, and so, I hear from the British Embassy, are thousands of other Irish-Americans, who are flooding its switchboard with sympathy calls and offers of service.

I haven't forgotten, mind you. I can recite, with a little prompting, all of Yeats' "Easter 19l6." I know in detail what Oliver Cromwell did to my people, particularly in Drogheda. And, like the speaker, I have lately been berating the prime minister for her inability to understand or appreciate the Irish.

But today, if I had a Union Jack, I would wave it.

My father was, in his youth, the president of the Hibernian Society. But he was a reader, and you can't read and be an Anglophobe. He never tired of recounting the wonders of his favorite novel, "Lorna Doone." He encouraged my brother and me to memorize the poems of A.A. Milne, "When We Were Very Young." We became involved in the affairs of the British royal family -- my father found them more palatable than the Irish-Americans in office in Massachusetts at the time. We followed closely the abdication of Edward VIII, which greatly troubled my father, who hated to see anyone shirk responsibility.

My affinity for the British even survived several visits to their island -- the understating, the overcooking. I revered Winston Churchill, but rejoiced at the liquidation of the British Empire.

But that was before the Falkland Islands. Churchill himself could not have been more furious.

Forget it is a corner of the world that people keep telling me isn't worth fighting about. Our rock-bottom principles -- against aggression, for self-determination -- our closest ally were under challenge. What's to be ambivalent about?

The Argentines didn't need to invade the Falklands to win my emnity. I have sat down with the mothers of the Plaza del Mayo, valiant women who march before the seat of government every Thursday afternoon, wearing kerchiefs with the names of their "disappeared" relatives embroidered on them -- and getting slammed around by the police for asking to know where their children are.

The government which lays claim to Ronald Reagan's consideration, has followed a policy of kidnapping its own citizens, torturing them, often to death, and of disposing of their bodies by dropping them from airplanes.

Yet Ronald Reagan, who instituted a "Remember Afghanistan" day, just couldn't make a choice between the country of the Magna Carta and the country of the "disappeareds." To me it was not just frivolous, it was obscene.

I watched the departure of the British fleet from Portsmouth with disbelief. I thought NBC was mistakenly doing a rerun of "Cavalcade" on the evening news. In the course of weeping and cheering with the the throng on the dock, I made an astonishing discovery: I am not really a dove. All during Vietnam, I screeched for negotiations. But as I watched the Invincible cutting the waters, I said, "Bash them."

I was pretty furious when I saw the pockmarked general that the Argies had sent to be the new governor-general of the Falklands. I thought of the poor islanders huddled in their houses, threatened with arrest if they put their noses out the door. I wish Alexander Haig well in his peace mission, but if it comes to military action, I regret to say that I am not flinching.

Thursday morning, my blood boiled again. I read in the paper where our U.N. ambassador, Jeane Kirkpatrick, had been on the night of the invasion. I know she fancies fascists, and dotes on Argentina as a model of a "moderately repressive dictatorship," but how could she be their guest of honor while the blood was still fresh on their hands?

When I found out where she had been, I realized that it had been simply out of the question for her to attend the U.N. session where Argentina was condemned the next day. It could have made dinner party conversation awkward. In conservative circles, it's okay to bash into someone else's country, but to disrupt a dinner party is simply unspeakable.

My authority on the gravity of the offense is conservative columnist William F. Buckley Jr., who some time ago wrote in severe condemnation of Jacobo Timerman, the Argentine editor, who was jailed and tortured for 30 months by Argentine officials, who didn't like his being a Jew. Apparently, he spoke of the folks in Buenos Aires in intemperate terms and ruined a dinner at Ms. Kirkpatrick's.

I hope that word reaches our Irish-American president in Barbados about just how strongly his Irish-American compatriots -- us new Anglophile-colonialist-hawks, that is -- feel about all this.