LONG BEFORE March 17, an advertisement appears in the personals column of the Irish Times:

"Celebrate the St. Patrick's Day holiday as only America knows how . . . You travel Aer Lingus Jumbo Shannon to New York . . ."

Obviously, you don't have to be Irish to enjoy St. Patrick's Day, and if it's corned beef and cabbage you're craving, you're definitely better off staying home. Ask for it in the old country, and they'll give you a shy-sly Irish smile and venture, "Oh, but that's an American dish really!"

If the Irish did concoct it, they probably waited 'til they got to Boston. The nearest thing you'll find to corned beef and cabbage in Ireland is bacon and cabbage. And the traditional food for Irish festive occasions is usually pork or ham.

Although Ireland's cuisine at its best can hold its own against any other country's, gourmet eating is not a preoccupation with the Irish. Yuri Ustimenko, Tass correspondent to Dublin in the '70s, wrote in his subsequent book, "Ireland": "Food for the Irish is not a pleasure but a severe necessity. The preparation of food is not an art which demands inspiration or creative impulse, but a daily duty from which there is no escape. Therefore, the Irish eat everything that is put on the table, unsqueamishly."

And with good reason. Not only is good food hard to find and exorbitantly priced, it is the very devil to prepare. In Connemara, the dramatically beautiful and rugged region on the west coast, country dwellers cook on tempermental coal stoves (the pungent peat fires on the hearth are reserved for the tea kettle and drying socks) that require brilliant engineering before a meal can be put on the table.

From winter into spring, the stores offer only a meager assortment of vegetables: brussels sprouts, carrots, cabbage and cauliflower are sporadically available and are frequently--to put it delicately--overripe. Potatoes, or "spuds," on the other hand, are plentiful and delicious. One variety, yellow-hued and floury, requires only butter, salt and pepper for a veritable feast. Local pubs pile disproportionate helpings of these potatoes on the plate; those unfinished are frequently carried off for supper.

Meat in Ireland is not aged as American meat is and has too strong and fresh a taste to suit our palates. The cuts can be unfamiliar, too; it's not unusual to discover that a pork chop has been wrapped around a cross section of the pig's kidney.

In the cities, there are often shops that only sell pork while others sell beef, lamb and poultry. The pork shops offer wonderful white and black "puddings," soft, spicy sausages actually, made of mysterious ingredients.

Shopping might be even more conveniently done, though, at the end of one's own driveway, when perhaps on Tuesdays at noon an itinerant butcher toots his horn, opens the back doors of his van, sets up a rickety scale and dusts off the tailgate which serves as his chopping block. "Mince" (ground) meat, chops and roasts are stashed in the back of the van while in the front seat is stashed the butcher's mother, apparently just along for an airing. Every request is met with a cheery "Right you be!" and when the week's marketing is complete, he stuffs everything together unwrapped into one too-small bag, chirps a pleasant, "That's the job now," hops in beside his Mum and takes off down the road.

Surrounded by rivers, lakes and the sea, in a region famous for its salmon fishing, you might anticipate that the fish would come when you whistled. But the only fish are likely to be found in the freezers of markets--until May, when it almost takes a bank loan to afford a pound of fresh salmon. It seems the winter and early spring weather is too unpredictable for the fishermen to venture out. This fickle run of the fish partially explains why the Irish did not turn to the sea to save them during the great potato famines of the middle 1800s.

Mussels, on the other hand, are in abundance. The government is experimenting with mussel farms in some of the harbors, but people simply slog through the seaweed at a nearby beach to pick them, finding also occasional bonuses of oysters, scallops and tiny curlicue snails called winkles.

The seashore also yields a white seaweed called carrageen, which can be boiled in milk with lemon rind and sugar to make a chalky tasting dessert of junket consistency. Better yet, boiled in water and eased down with whiskey, it is a traditional remedy which makes an Irish cold a pleasant affair. In addition to dessert and cold medicine, carrageen is recommended for stomach ailments, whopping cough, arthritis, foot troubles and shampooing.

This week, in our all-conveniences American homes, we can toast ourselves with a glass of syrupy, tepid Guinness, the way the Irish like it, watch the New York Irish parade across our televisions and enjoy the following St. Patrick's Day dinner.

LEEK AND OATMEAL SOUP (6 servings)

This is a traditional Irish soup, delicious and full of fiber. 4 to 6 large leeks 2 tablespoons butter 2 1/2 cups milk 2 1/2 cups chicken stock 2 tablespoons oatmeal, uncooked 2 tablespoons chopped parsley 1/2 cup cream to garnish Salt and freshly ground pepper

Wash leeks well and trim, leaving most of the fresher green parts. Chop into 1-inch pieces. Combine butter, milk and stock in a large saucepan. When boiling, add oatmeal and boil 5 minutes. Add leaks and seasoning; cover and simmer for 30 minutes. Add half the parsley and cook 5 minutes. Serve with a little cream in each bowl and sprinkle with parsley.

CABBAGE STUFFED WITH HAM (4 servings)

This recipe originated in the fine Dublin restaurant, Snaffles. 1 medium cabbage 1/2 pound soft white bread crumbs 1/2 cup milk 1/2 pound cooked ham, minced 1/2 pound bacon, finely diced 2 tablespoons total parsley, chives and tarragon 1 shallot, chopped Salt and pepper 2 egg yolks, well beaten 1 1/2 cups stock

Boil cabbage 15 minutes and drain, spreading out leaves from core. Prepare stuffing by mixing bread and milk, pouring off excess milk. Add ham, bacon, herbs and shallot and season with salt and pepper. Add egg yolks. Put a little stuffing between each leaf and the rest in the center. Reform cabbage and tie with string. Place in deep pan, add stock and simmer, covered, for 3 hours.

POTATO SCONES (Makes 10)

The following recipe is a great way to use leftover mashed potatoes. They don't rise much, are crispy on the outside and soft inside. 2 tablespoons butter 1 cup flour Dash of salt 3/4 cup mashed potatoes 3 tablespoons milk 1 1/2 teaspoons baking powder

Mix butter into flour and add salt. Mix in mashed potatoes and enough milk to make it soft and pliable, but not damp dough. Roll out on a floured board and cut into 3-inch rounds. Bake on greased baking sheet at 450 degrees for 20 to 30 minutes.

SYLLABUB 2 lemons 4 tablespoons sherry 2 tablespoons brandy Pinch of nutmeg 4 ounces superfine sugar, plus sugar for cream Sprig of rosemary 1 pint heavy cream

Grate the peel of lemons, then squeeze the juice. Mix with sherry and brandy, nutmeg, sugar and rosemary. Cover and let stand a few hours. Strain into 6 to 8 tall glasses equally. Whip cream, add sugar to taste and divide over tops. If you prefer, you can whip the cream into the liquid.

WHISKEY PARFAIT 4 eggs, separated 2/3 cup confectioners' sugar 2 tablespoons Irish whiskey 2 ounces flaked toasted almonds 1/2 cup heavy cream, whipped 1 ounce mixed peel, grated

Put egg yolks and sugar into large bowl; beat well with a wooden spoon until light and creamy. Stir in whiskey and almonds. Fold cream and stiffly beaten egg whites into whiskey mixture. Blend gently but thoroughly. Rinse a 2-pint mold or ring mold with cold water. Sprinkle mold with mixed peel and pour in mixture. Chill well. Turn out by dipping the mold in boiling water for a second.