THE IRISH EMBASSY ambassador's residence, with its semicircular Greek temple portico, sits with dignity softened by fall flowers on one of the most handsome blocks in Washington, the 200 numbers of S Street NW, just above Massachusetts Avenue.

The Irish like to think the house looks like a rosy brick Georgian house in Dublin. But the house has sheltered the Irish only since 1966. It is the first Irish ambassador's residence to be owned by the embassy. Before, the Irish Republic rented houses.

Former ambassador and Mrs. William Fay, who had rented the nearby home of former ambassador and Mrs. George Renchard, chose the house after some years of searching. The Fays decorated the house with the French antiques. Mrs. Fay, a concert pianist, had been educated in France, and he had served as ambassador in Paris.

Ambassador and Mrs. Sean Donlon have recently remodeled and redecorated the house in a more modern and comfortable style to show off the best of Irish art, furniture, glass, china and fabrics.

Using Irish furnishings is in keeping with the new wave of embassy design emphasizing each country's own culture and taste. In the past, embassies, slavishly followed the old rule which said that French was the language of diplomacy, and Louis XIV (more often FFF or fake French furniture) was the only embassy-decorating style.

"The redecoration took 18 months," said Mrs. Donlon. "We had to move out for six months."

The other day, as she talked about the embassy, she was almost covered up with Irish wool throws, table runners, wool pillows, Belleek china, and Waterford glass. The crafts were brought by Bloomingdale's to show her the Irish wares in the store's six-week Irish promotion opening today.

"We oversaw the new interior design for the residence," said Mrs. Donlon over coffee and cookies "which was executed by the Irish foreign office design staff."

We had come into the ambassador's residence on a bright fall day, admiring the begonias edging the house, contrasting with the stone trim and matching the brick. The house is a charming, classic-revival building, with tall ceilings on the first floor and spacious lawns overlooking the city. It is a cozy house, if 20 rooms can be said to be, with none of the marble halls or waltz-size ballrooms of the old embassies.

The vestibule has, as you'd expect, a Waterford chandelier, one of two in the downstairs public rooms, sending prisms of light over the carpet, an unobtrusive dark beige. It hides a black and white tile floor. "When the carpet wears out," said Mrs. Donlon hopefully "we can go back to the black and white."

The entry hall has a bowl-shaped lighting fixture similar to Lalique glass, showing classic dancing maidens.

On the front hall wall is the first of three plaster heads, cast from the ornamental keystones carved for Gordon's Custom House in Dublin. The heads are river gods, representing the Rivers Shannon, Lea and Lagan, with headdresses ornamented with pitchforks, snakes and flowers.

The drawing room and the dining room stretch across the entire south side of the house. The house is good passive solar design, with the tall windows facing south, smaller ones on the north and a minimum number on the east and west. The floor-to-ceiling windows open, like those in the Blue Room of the White House and at Monticello, high enough to walk through.

The windows give promising glimpses of the stone-edged south terrace where the Donlons often eat in pleasant weather to admire the brilliant geraniums and begonias edging the rose bushes in the garden.

The swimming pool, having sprung a leak, was empty, but the landscapping service was busy digging up the flower garden around it to replace the damaged planting.

The drawing room is a long thim shape climaxing in a fireplace flanked by two glass cabinets. The dark green background of the glass-front, Palladian-shaped cabinets, sets off the embassy collection of Waterford glass and Belleek china.

The white walls of the drawing room are ornamented with an elaborate plaster cornice and wainscoting. The Irish-made furniture is comfortable -- soft modular units covered in a black velvet, organized into two units set catty-corner. Tub chais in tweed fill in the other corners. Each group is arranged around steel and white granite circles, topped with small Waterford cut crystal ashtrays and bowls.

A crystal jar holds green jelly beans, a gift from President Reagan when he lunched at the residence last St. Patrick's day. The Donlons try to discourage people from eating the presidential gift, but they're about half gone. At the same lunch, the president received a scroll prepared by Debrett tracing his Irish ancestry as well as an Irish china basket and a bowl of Irish silver.

Most of the paintings in the room are handsome contempory works by Irish artists. Three wool tapestries on one wall by Helen Ruth look Irish and handsome. The room is lighted with elaborate recessed spots. Natural colored Irish linen curtains hang at the windows.

The dining room is traditional with Sheraton sideboards with inlaid decorations and a long mahogany table and matching chairs. The paintings are on loan from Ireland's National Gallery in Dublin, most by 18th- and 19th-century Irish artists. The painting of Venice over the black marble fireplace is echoed in the screen painted with Venetian scenes. Not only is the chandelier Waterford, but also a handsome large flower bowl. A large Irish silver candelabrum is often used on the dining table.

The coziest room is the library, off the front hall. Three big soft leather sofas in dark brown make a U around the fireplace. The two bookcases flanking the fireplace are full of the Donovan's own collection of books and records "Books and sheet music are really what we collect," Mrs. Donovan said.

Across one wall is a grand piano, a Yamaha, "They were made for a time in Ireland," Mrs. Donlon said. All the Donlons are musical. The 16-year-old daughter, Monica, sang at the lunch for President Reagan.

On a console table is a collection of Irish crystal decanters.

Major changes upstairs included a new staircase and linen rooms remodeled and decorated with Kilkenny Design Workshops wallpaper and fabric to make bedrooms for Monica and the 11-year-old son, Brendan, as well as a three-room servants suite.

The Donlon family will be going back to Dublin in a few weeks. "We had hoped to stay until next spring so Monica could finish her senior year," said Mrs. Donlon. "We've already been notified of our transfer. But we will be happy to be back in our cottage near Dublin."

The Bloomingdale promotion isn't the only Irish event coming up. The postmaster this month is issuing a stamp honoring James Hoban, the original architect of the White House. Hoban, an early architect and builder, was inspired by (harsher ones would say "copied") houses he was familiar with in his native Dublin, and portrayed in James Gibbs' "A Book of Architectue." Hoban had been trained at the Dublin Society architectural school, and worked on a number of Dublin buildings before he came to Washington.

A recent book by Irish architectural historian and ale heir Desmond Guinness speculates that Hoban took his plan not only from Leinster House in Dublin, as another architect Benjamin Latrobe charged, but elements of several other Irish houses as well.

The Irish Embassy is not nealy so grandiose as its seniors, thse magnificent houses of Dublin and environs. The house was designed by Waddy Wood, who also did the Woodrow Wilson house, the Alice Pike Barney and at least 14 other houses in the Kalorama area. The Barney house in 1902 was one of the first to be built in what was then called Kalorama Woods.

The Kalorama area takes its name (Greek for beautiful view) from the estate of Joel Barlow. In 1807, with the achitectural help of Benjamin Henry Latrobe, Barlow rebuit and added to an 18th-century house to make a mansion. Thomas Jefferson came often to visit Barlow, a diplomat and poet.

Kalorama mansion, at what is now 23rd and S streets, according to "Capitol Losses" by James M. Goode, was demolished in 1889 so the city could cut S Street through from Florida Avenue to Massachusetts.

The Irish residence at 2244 S Street was built for $75,000 in 1924, the most expensive of all Waddy Wood's Kalorama designs, according to "Waddy Wood in Kalorama," a monograph by Emily Hotaling Eig and Gray Bryan III, the authoritative work on the subject.

The original owner was Frederic Delano, an uncle of Franklin Delano Roosevelt who lived in Washington when he was a member of the Federal Reserve Board.

Wood, a Virginian and a self-taught architect, is responsible for an amazing number of Washington houses not only in Washington but in the Virginia Hunt County. Wood's own country home, Leeton Forest, is now owned by Charles Seilheimer, president of Sotheby, Parke Bernet International Realty Corp.

Wood also designed the Capitol Traction Company Car Barn at Key Bridge and M Street in Georgetown and the U.S. Department of the Interior building at C Street between 18th and 19th Streets NW.