Last week, 1,521 years after St. Patrick supposedly banished snakes from Ireland, an 18-inch anaconda hitchhiked into the country on board a bunch of South American bananas and was discovered--alive--in a County Kildare grocery store. The tiny reptile, which was featured prominently in the Dublin daily papers, was at last report residing in the city zoo, munching on worms and clearly ignorant of its untimely arrival and the saint's legendary antipathy toward snakes.

The snake evoked one of the few mentions of the saint himself in a country that has diligently been Americanizing St. Patrick's Day.

"St. Patrick's Day used to be a religious feast day, not anymore . . . it's becoming too commercialized," lamented one Dublin librarian as he searched through his files for yellowed newspaper clippings of St. Patrick's celebrations of years gone by.

Indeed, until the last decade or so, "Patrick's Day," as it is commonly called here, was a quiet, stay-at-home holiday. Schools and businesses closed, but so did the pubs, so for most people it was off to mass in the morning, then home to spend the rest of the day with the family. For many years, the only place drink could be had on March 17 was at the Irish Kennel Club dog show in suburban Dublin, which was thought to account for that show's once-enormous popularity.

Those days are gone forever, say Irish tourism officials, as they rub their hands with glee over the success of their campaign to "Americanize" the Irish national holiday--and thus attract more tourists.

Dublin now celebrates St. Patrick's Day in an increasingly American style. There is the annual St. Patrick's Day parade, which has grown from a drab display of a few commercial floats to a flashy pageant boasting 90 entries, including three American high-school bands, one high-kicking American drill team and at least one American fife and drum corps. Racks of St. Patrick's Day cards by Hallmark are displayed prominently in the city's shops.

In addition, there are open-air concerts around the city both before and after the parade, plus the Lord Mayor's ball tonight at the celebrated Burlington Hotel and a public ceili (celebration) at the Lord Mayor's official residence.

Today Dublin's pubs will be open--albeit on Sunday hours (closing time is 10:30 p.m.)--and most publicans are expecting record business because the price of a glass of beer will jump 21 cents tomorrow morning, bringing the price to over a dollar. Several pubs have even begun to advertise that limited amounts of green beer will be available in their establishments for the first time tonight.

Not exactly New York City, where by nightfall the streets normally are littered with debris, broken bottles and drunken revelers. But they're getting there.

Surprisingly, the move away from the traditional religious holiday has not been decried by Dublin's Catholic clergy, who insist that the average parishioner still goes to mass before he heads off to the pub.

"St. Patrick is sort of a symbol as well as a religious figure," said Father Michael Sweetman, a senior priest in one of Dublin's inner-city parishes. "I think it is still regarded as a day of religious obligation so I certainly don't think the pageantry and celebration take away from the holiday.

"I think it's a very nice thing to see all those pretty girls in parade, although I must say they look like they must be freezing to death," he added.

For many Irish people, intrigued with stories of raucous American St. Patrick's Day celebrations, the change is not fast enough. As a result, nearly 1,000 Irish tourists left Ireland this week to spend their national holiday in the United States. Aer Lingus, the national airline, has mounted a low-key campaign complete with cut-rate transatlantic fares and discount accommodations to lure Irish tourists to America.

"Celebrate St. Patrick's Day with the Sons of Shillelagh" and "Celebrate St. Patrick's Day as Only America Knows How," beckoned Aer Lingus advertisements in the pages of The Irish Times during the last several months. And, last Saturday: "Last-Minute Chance to Sample the Green Beer."

"It's like the difference between whiskey and water," said Sean Carberry, a spokesman for the Irish Travel Agents Association, when asked to compare the dull Dublin festivities with New York merrymaking. "New York makes St. Patrick's Day into a party, a fiesta, while Dublin has a pretty restrained atmosphere."

The departure of one particular Irishman, however, the newly elected Irish prime minister, Charles J. Haughey, has been met with widespread criticism here. Haughey, in the U.S. to lure American industry to Ireland, is lunching with Ronald Reagan at the White House today.

The leader of Ireland's Labor Party, Michael D. Higgins, was quoted last weekend as condemning Haughey's trip. Higgins said it was "unthinkable" that the leader of the Irish parliament--which has unanimously condemned the "repression, killings and torture" in El Salvador--should spend St. Patrick's Day with the leader of a country so strongly associated with the Salvadoran government. Higgins, who recently returned from a fact-finding trip to El Salvador, called Ronald Reagan "the single greatest threat to world peace in our time."

Additional criticism has come from several Irish students' groups which launched petition drives asking Haughey to remain in Ireland for St. Patrick's Day as a means of protest against American involvement in El Salvador.