I SUPPOSE THE reason they asked an Irishman to preview "The Best of Families," the new TV series that is partly about an Irish tenement family,was that they were hoping for something cheerful with a lot of shuur an begorras in it.

They ain't gonna get it from me.

This "major new television dramatic series which colorfully blends American history and fiction" gives itself a two-hour running start Thursday at 9 p.m. on PBS. It juggles three families the destitue lower East Side Raffertys, the railroad-rich Fifth Avenue Wheelers and a lovebound young Congregationalist couple: she from Brooklyn, he from Indiana.

The first thing that happens is that John Houseman starts telling us all about "the program you are about to see." In televisionese, this means: Important Event, Everybody Sit Up. At least Houseman doesn't simper like Alistair Cooke, and in his competent way he feeds us some useful predigested information about the time concerned, 1880 to 1900.

But I do hate to be harumphed at by a television set.

Oh, there's plenty of history here. A full-time team of six researches, dozens of consultants and 14 "distinguished advisers" has given us authenticity right down to the silverware. In fact, history is injected into the story the way mold is injected into domestic blue cheese. You can see theneedle marks.

The first words in the show are a newsboy's: "Get yor Sun, your New York Sun!" Surely the people of New York wouldn't have had to be told it was the New York Sun and not the Baltimore Sun. Next we get a whiff of politics as someone mtones with studied offhandedness, "We at Tammany Hall say . . ."

If someone bears an old injury, it's not just any old injury but something he got working on the Brooklyn Bridge. When someone talks of finding work upstate, it's not just any old work but work on the canals there. We're drowning in history.

I felt that these people weren't acting so much as reenacting. It' not a drama; it's a pageant.

For some reason I am reminded of that unfortunate French play where the doctor rushes in after delivering a baby and shouts, "It's a boy! And his name is . . . his name is . . . VICTOR HUGO."

There are many fine things about this show, painstaking sets either elegant with the glow of candlelit silver and crystal or noisome as only a tenement can be (though it does seem an unusually airy tenement to me), and several first-rate set-pieces: a wake, a bare knuckles prizefight, a saloon opening and so on.

The trouble is, I was never moved. Now and then the actors broke through their respect for the show itself to achieve passion but, in the first session at least, there hadn't been time to build the solid emotional identification that can tear up a viewer.

Last week I saw another two-hour documentary-style drama, also on PBS, Robert Young's "Alambrists!" about a young Mexican illegal immigrant. It was episodic. It was confusingly cut and sometimes baffling. Yet that Mexican kid got to me as none of these people ever did.

I think the reason is that when television wants to cover a lot of ground fast, as it evidently feels it must for these grandiose spinoffs from "The Forsyte Saga," it falls back on shorthand devices. Some of these are as old as the Silents; some are TV soap opera conventions.

Shortly after he comes on, the rollicking patriarch Patrick Rafferty (the brilliant and unappreciated Milo O'Shea) coughs a certain dry cough. That is the Death Cough. He might as well have a skull and crossbones painted on his forehead. So when, after a whole series of such signals, he does die, we are mainly relieved.

Furthermore, he doesn't get much time to rollick. Only O'Shea's skill provides us with some suggestion of the man's character beyond the script.

Signals. You want to say these folks are Irish, all right, you give them all brogues (O'Shea's burred accuracy makes most of the others sound amateurish). And you have someone do a clog dance in a saloon. But do you really have to make the guys smoke clay pipes, too? And must they be hod carriers? And can't we have just once an Irish drunk who doesn't rollick?

Signals. Every time we enter a rich house (or bank) a clock chimes somewhere in the refined hush. Every time a new scene starts, a horse and carriage clops past the camera. It's 1380, remember? Oh, yes. Right.

Certainly they're afraid we won't understand a thing unless it is beaten into the ground with explanations and demonstrations. Some men at the wake are roaring and singing and waving bottles. "They're drunk," a girl explains.

Lovers-to-be exchange glances. That's how you know they are lovers-to-be. Widows look sad. Rivalrous brothers scowl. Venal bankers squint. Rebellious sons sneer. Disgusted barmaids look disgusted. It's about as complex as Dick and Jane.

Perfunctory is the word. Even when the loves joyously kiss in the summer rain, I couldn't care less. It's like the old gag about the people who had heard the same jokes so often that they numbered them and would laugh when someone simply called out the number.

I keep remembering a scene in "Alambrista," where a counter girl studies the face of the forlorn youth she has brought home from the cafe to shelter him for the night, and she is not really deciding to go to bed with him, not yet, but she is considering. She is also slightly afraid. And anxious about her kid brother in the next room. And wondering what she might be turning into.

It was real. That whole movie glowed with authentic human feeling. Time and again I found myself wondering how many of these people were nonactors.

In "The Best of Families" everyone acts. All the time.

It is one thing to try to show what happens to someone, how life changes him, how he affects a world he has stumbled into. It is something else to feed our preconceptions, to tap us for emotional kneejerks, to reduce life to a series of familiar illustrations through which move simplified characters exhibiting expectable reactions if not feelings, speaking in reassuring cliches. (Surely the word "credibility" wasn't in vogue then?)

I frequently hear these ponderous panoramic series defended on the basis that they are so much better than the usual TV pap, that they are a noble attempt to rise above the daytime soaps, that they are the best we can expect from television.

I might believe it if I hadn't seen "Alambrista."