The last time Old Town Bar celebrated a major anniversary, it was the 100th birthday of its colossal porcelain urinals in 2010.

“It was distinctive, it got people’s attention,” co-owner Gerard Meagher, 65, said of the festivities, which involved Champagne. “Plus, they’re really nice.”

Meagher and his four siblings, second-generation owners of the building at 45 E. 18th St., feted its 125th year as a watering hole for thirsty New Yorkers on Sunday. (During Prohibition, patrons still drank at what was then called Craig’s Restaurant; they hid their hooch under wooden bench panels when the cops paid a visit.)

Old Town celebrated its longevity and German roots with a Bavarian oompah band, sauerbraten and period costumes.

Amid the churn of the New York City bar and restaurant scene, as the city’s most-beloved institutions throw their wildest parties just before rent hikes or the stress of maintaining a business shutters them for good, the bar's secret to long life is property: "Owning the real estate is the [key] unfortunately," said Meagher (pictured below).

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"We've seen a lot of beautiful places [with heritage] knocked down," he said. "We'd just like to retain a slice of old New York where there’s no elite, everybody is judged by the content of their characters, not by their status or their credit rating."

Bartender Patrick Lydon (pictured below), 31, the youngest of his co-workers and still a veteran of 11 years, compares Old Town to Union Square.

“Just like Union Square is the New Yorker’s main transportation hub, we’re like a longtime place where anybody’s welcome,” Lydon said, adding that the bar’s clientele ranges from “regulars that have been coming in here longer than I’ve been alive” to tourists to “younger people who’ve just discovered it and like to say they have discovered it.”

A NEIGHBORHOOD BAR

The fortunes of Old Town, and its vicissitudes, have tracked those of the neighborhood.

In 1892, the bar opened on the outskirts of what was then the “Ladies’ Mile,” a shopping district for the well-to-do, as the Reichnecker Cafe, a German bar serving liquor and a “business men’s lunch” from 11 a.m. to 3 p.m. (Its beloved original fixtures — the mahogany bar, the beveled mirror behind it and the mosaic tile floors — date to this period.)

The cafe became Craig’s, a speak-easy masquerading as a restaurant, in the 1920s, then Old Town in 1933, under a new proprietor named Claus Lohden.

In 1948, the closing of a nearby subway station on 18th Street stunted the neighborhood and the business. Real estate was cheaper uptown, so commercial businesses migrated north. The “whole area went downhill” over the next two decades, Meagher said, the park itself becoming a hot spot for the illegal drug trade. Old Town contracted, with Lohden’s son Henry closing the upstairs dining room.

The bar’s revival came in the late 1970s, when a newspaper copy boy-turned-Brooklyn bar owner asked for the opportunity to restore, in Meagher’s words, a “great place, down on its luck.”

“It was only open 9 to 5, Monday through Friday,” Meagher said of what was then a stopover for truckers (and an occasional hangout for Andy Warhol, who visited from his nearby studio).

“So my father [Larry] said, “Let me come in, let me bartend and open up at night and see if we can get this place back up on its feet.”

Larry Meagher — to whom Henry would pass along the business in his will — made his greatest mark on Old Town just as the neighborhood underwent a redevelopment renaissance: In 1976, one of the city’s first Greenmarkets set up shop in Union Square Park; in 1984, the business improvement district the Union Square Partnership was formed; and in 1985, the city renovated the park, creating a new plaza at its south end, and rezoned the block to the east for high-rise condos.

The new Union Square attracted foodies, real estate developers, middle class residents and NYU students from the university’s expanding campus.

At Old Town, Meagher did his part to cultivate an ambience that was at once low-key but worldly. His plain burgers — transported to this day from the second-floor kitchen to the bar via a working dumbwaiter — were “everyday, blue-collar burgers,” as Lyndon described them. His bar served beer, wine and spirits. The working men still came for their lunches.

THE OLD TOWNERS

But Larry Meagher, who died in 2007, took pride in the literati who visited his establishment, the likes of Frank McCourt, Seamus Heaney, Billy Collins, Christopher Hitchens and Nick Hornby. He donated $1000 to the neighborhood’s fledgling Irish Repertory Theater, plucking the money out of the mattress in his third-floor apartment and handing it to co-founder Ciarán O’Reilly.

“It’s my favorite donation that we ever got,” said O’Reilly, who brought his wife to the bar for their first date.

Old Town has welcomed its share of TV and film productions, too, the posters and images hanging on its walls testifying to the cinematic appeal of its old New York character and high ceilings: the opening sequence of the TV show “Mad About You,” the 1990 film “State of Grace,” Madonna’s 1993 video “Bad Girl" were all shot in the bar.

With the second generation of Meaghers, the bar has opened its doors to employees of the neighborhood’s new media companies, like BuzzFeed, and banned smoking and cellphones. (Gerard Meagher at first resisted then-Mayor Michael Bloomberg’s smoking ban, but he changed his tune when he realized it increased patron turnover and the mayor’s staff were regular customers.)

“This is a place where you can still talk,’” Gerard Meagher said, paraphrasing McCourt. “We don’t have — except when Patrick’s working — blaring music, and we don’t have 30 TVs.”

“We encourage that sort of down-to-earth, intelligent talk, and not just sports and gossip.”

On a Sunday afternoon, the bar packed with costumed revelers, that talk was all about the bar itself.

“This institution has been located in our neighborhood for a remarkable 125 years, and as Union Square continues to evolve, Old Town Bar represents a piece of the historic district,” said Union Square Partnership’s executive director, Jennifer Falk.

“It’s a wondrous place,” O’Reilly said. “There are very few others that have the history, the ambience of something that’s been around that long in New York."