A level below the ornate ballroom at Capitale, a 19th-century beaux-arts events space in Manhattan, Cathal Armstrong is pounding a bottle of San Pellegrino mineral water. His face is covered with a thin sheen as beads of perspiration work their way down his forehead and cheeks. It’s hot as a Bronx subway platform in August in this subterranean kitchen.

Armstrong has just finished a task that he would never, not even in his most fevered dream state, undertake at Restaurant Eve, his intimate, airtight operation in Alexandria: He was part of a production line that plated about 350 entrees of braised lamb shoulder with medallions of roasted loin. Armstrong gave himself the job of placing the slices of seared, pepper-crusted loin atop the braised meat and root vegetables. They knocked out those entrees in about 20 minutes.

Now Armstrong looks wiped, but in that pleasurable way in which the body is tapped out but the brain is flushed with endorphins. The chef seems almost giddy. Or as giddy as this dry, driven Dublin native gets. “It’s exhausting. It’s fun,” Armstrong says between chugs of mineral water. “It’s not every day, thank God.”

You might be wondering why a four-star chef and multiple James Beard Foundation Award nominee would put himself through such banquet hell. The answer is simple: Armstrong wanted to prove that Irish cuisine isn’t just stews, colcannon, boxty, chips, fadge and all those other dishes with potatoes coming out of their ears. The Archaeological Institute of America asked Armstrong whether he’d be game to create his interpretation of an ancient Celtic dinner for its annual gala, and the chef gladly accepted.

“It was my idea, because I’ve been to his restaurant,” says Jennifer Klahn, the AIA’s director of development. “It was absolutely delicious.”

Even though Armstrong agreed to create the meal for the AIA’s annual fundraiser — which can generate a cool half-million for the country’s oldest and largest archaeological organization — he could have done what other chefs had before him: just consult on a menu and never show up for the actual event. Guest chefs for the previous two AIA galas, which featured Mayan cuisine and the foods of Peru, were not asked to make an appearance, because Capitale requires its kitchen to cook all meals. But Armstrong was not about to accept those conditions. He asked for and was granted an exception to the rule. He had learned his lesson in that regard.

Last year, at a Jewish Council for the Aging program honoring The Washington Post’s retired food critic Phyllis Richman, Armstrong and other prominent D.C.-area chefs were asked to contribute recipes for early-evening hors d’oeuvres. He handed the cooks at a local hotel the instructions for his leek mini-tarts. He can’t remember what they did with his dish, or even whether they cooked it, but he recalls watching in horror as plate after plate of mediocre banquet fare was ultimately served to the best culinary minds in the region, including Bryan Voltaggio of Volt and Michel Richard of Citronelle. The evening left an impression.

The chef had no intention of placing his reputation in the hands of others for the AIA dinner, where the master of ceremonies would be Gabriel Byrne, an archaeologist-turned-actor and now official cultural ambassador for Ireland. “Any time I’m going to put my name on something, I’ve got to make sure it’s hands-on,” Armstrong says as he methodically spoons fennel puree on a sea of white plates that cover tabletops in two rooms in the Capitale basement. “It’s my food.”

Several weeks before the gala, Armstrong had made a trip to New York to inspect the Capitale kitchen and meet its 35-member crew, headed by executive chefs Osvaldo Garrido and Jason Munger. At Capitale, the chef found next to nothing that would limit him creatively. Instead, the limitations would come from the sheer scale of banquet cooking. Armstrong needed to plan a menu that could be largely prepped ahead of time and would avoid ingredients that ran afoul of the many dietary restrictions among the well-heeled guests. The AIA had forwarded the chef a list of potential ingredients for the ancient dinner but left it up to Armstrong to develop the menu.

Armstrong, only half-seriously, was tempted to place platters of spit-roasted rabbit, birds and wild boar in the middle of the banquet tables, but he realized the absurdity of asking black-tie guests to break off hunks of meat and eat with their hands. Instead, he opted for less dramatic approaches to authenticity: For one, he banned the potato from the menu, if only to dodge cracks such as the one Colman Andrews reprises in his book “The Country Cooking of Ireland” (Chronicle Books, 2009): “An Irish seven-course dinner, went the old joke, was a potato and a six-pack of Guinness.” In the same vein, Armstrong said, “I didn’t want to do salmon.”

Instead, he devised a concise three-course meal that opened with a smoked mackerel salad with wild garlic, dandelion greens, fennel puree and pickled fennel. (“In medieval Ireland,” noted the menu description, “fennel was hung over doorways on Midsummer’s Eve to ward off evil spirits.”) The lamb course followed, served with barley wildflower-honey cakes. (The menu again: “. . . in ancient Ireland bees were associated with wisdom and were thought to be the messengers between our world and the spirit realm.”) Dessert was a trio of custards — mead, wild berry and apple-caramel — and tiny squares of shortbread. (“In Celtic mythology, it was said that a river of mead flowed through paradise and apples were used as a symbol of fruitfulness and healing.”)

Armstrong swears that “the vast majority” of ingredients could have been found on the Emerald Isle back in the 4th century, way before the spud made its way to Ireland around the 16th century and then dominated the country’s cuisine, for better and for worse. “They didn’t have micro-arugula,” he says, “but they certainly had lamb, parsnips and carrots.” But if Armstrong’s ingredients were rooted in the past, some of his techniques, such as his sous-vide approach to preparing fennel, were clearly entrenched in the 21st century.

A couple of days before he left for New York, Armstrong vacuum-sealed five pounds of fennel bulbs under the maximum pressure his Koch machine could apply, then cooked them in a water bath at 181 degrees for 45 minutes. “You get a more concentrated color and more flavor,” he said that afternoon at Eve.

Armstrong would have preferred to sous-vide the carrots and parsnips in his lamb entree, too, but he didn’t have the capacity at Eve to cook the 50 pounds of root vegetables needed for 350 servings. Instead, he sauteed the carrots and parsnips in Kerrygold butter at Capitale. He also personally seasoned and seared each and every lamb loin at the New York venue, after chauffeuring the grass-fed Virginia lamb to the Big Apple himself.

He did leave some work for the New York crew: He had more Virginia lamb shipped to Capitale, where Garrido and staff braised the shoulder meat for hours. The Capitale crew also got a rude surprise on April 26, the day of the gala: The mackerel fillets arrived late that morning — and with the pin bones still in them. A crew of four, including Armstrong, spent more than four hours cleaning, trimming and wood-smoking that fish.

Both Garrido and Capitale executive pastry chef Sarah Buck found Armstrong laid-back even under the trying circumstances of banquet cooking and late-arriving ingredients. “He’s a very mellow chef,” says Garrido.

Up in the dining room, the official praise from the dais bordered on blarney. “Well done, Cathal,” noted Joe Byrne, the executive vice president of Tourism Ireland. “You did us . . . proud this evening.” The graying and bowtie-less Gabriel Byrne also had gracious things to say about Armstrong, calling him “an exquisite chef,” even though the actor had not yet tasted his meal — and hadn’t visited the chef’s restaurant in Alexandria. The actor said he’d sampled Armstrong’s food while the chef was prepping the previous day.

Whether that’s true was beside the point. This event was about rooting for the home team away from home, the Irish in America pulling for Ireland, that picturesque isle too often reduced to the land of leprechauns, Guinness, U2 and potatoes. In his way, Armstrong was one more cheerleader, personifying both past and future: a chef who embraces modern technology and French techniques but who knows that local, natural, farm-to-table ingredients are just part of his nature. Ireland, after all, never has had much taste for Big Ag. In this meal, you could see the beginnings of a modernist Irish cuisine, simultaneously ancient and forward-thinking.

At least one impartial diner was impressed. Marshall Heyman writes the Heard & Scene column for the Wall Street Journal. He attends parties . . . well, he attends a lot of parties and banquets every month and isn’t one to pass out idle praise. “It was definitely one of the better meals [I’ve had],” Heyman said, “if not one of the top five.”