Jean Kennedy Smith’s grand Manhattan duplex overlooking the East River, where she played host to her large, prominent family, along with luminaries from political, art and showbiz circles, is being sold by her estate. The asking price is $6.45 million, with $19,975 in monthly maintenance.

Ms. Smith, who was the last surviving sibling of President John F. Kennedy and the second-youngest in the Kennedy clan, died last June at the age of 92. She served as the U.S. ambassador to Ireland in the 1990s and was credited for helping end decades of violence in Northern Ireland. In 2011, she was awarded the Presidential Medal of Freedom.

Her apartment is a combination of two units on the sixth and seventh floors of 4 Sutton Place, a 14-story co-op ensconced in a leafy enclave lined with 19th-century townhouses. The nine-unit, brick-and-limestone building, between East 57th and 58th Streets, was designed by the architect Rosario Candela and erected in 1928.

Ms. Smith bought the duplex in 1997 and moved in a year later, after completing her diplomatic service, according to Amanda Smith Hood, one of her four children and the owner of a small equestrian-goods business in Washington. She acquired a two-bedroom adjoining residence on the seventh floor in 2001, she said, and used it mainly to house guests.

“She had a lot of Irish friends who came to visit,” Ms. Hood said, among them, Seamus Heaney, the poet and playwright, and Bill Whelan, the composer. Her other close friends included the actress Lauren Bacall and the Broadway veterans Phyllis Newman and Adolph Green. “She tended to gravitate toward artists,” her daughter said.

Ms. Hood recalled how her mother especially enjoyed hosting gatherings at the apartment. During the holidays, she said, “she would have two overlapping parties: a smaller kids’ party with magicians and Santa, where she would invite friends’ children and grandchildren and the doorman’s children, and an hour later would be the grown-up party. It was a great space for entertaining.”

The home encompasses about 5,680 square feet and contains four bedrooms and four and a half baths in the main duplex portion and two bedrooms and two baths, with a kitchen and sitting room, in the connecting apartment.

The prewar layout remains largely intact, with mostly cosmetic changes done over the years and renovations to the main kitchen and bathrooms. Many of the original architectural flourishes also remain, such as the hardwood floors, nearly 11-foot-high ceilings and crown moldings.

The formal entrance is through a private elevator vestibule on the lower level that opens to a spacious central foyer, where a curved, windowed staircase sits. The foyer leads to an enormous living room, a cozy library/study and the dining room.

“That’s a signature of Candela — he’s known for his grand staircases and for his palatial, grand living rooms,” said Paul Kolbusz, a broker with the Corcoran Group who is listing the property with his colleagues Markus Buchmeier and Melissa Sargeantson.

The 20-by-28-foot corner living room is anchored by a wood-burning fireplace with an ornate granite and marble mantel brought over from Ireland. The room features an imposing crystal chandelier and three Juliet balconies that offer scenic views of the river and Sutton Place.

The nearby library/study, painted a dark red, has built-in bookshelves and a fireplace with a carved wood mantel, also from Ireland. “The red room was where we had drinks and cocktails before dinner and then would later go back to and talk or watch a movie,” Ms. Hood said.

The formal dining room next door is also lined with shelves and has three more Juliet balconies. It leads to a large eat-in kitchen equipped with stainless-steel appliances and wood countertops. Two large windows look out onto the Queensboro Bridge and the wood floor has a painted faux rug.

Beyond the kitchen is a guest bedroom with an en-suite bathroom and a staff bedroom and bathroom. There is also a powder room and laundry room.

The other bedrooms are on the upper level. The primary suite has another wood-burning fireplace, along with two balconies, a large walk-in closet and a bathroom. It also shares another bathroom with a neighboring guest bedroom.

Entry to the guest apartment is through a gallery off the seventh-floor elevator vestibule.

“What is the most impressive feature of this duplex is the grand proportions of all the rooms,” said Mr. Kolbusz, the Corcoran broker. “It’s elegant and chic. The apartment really defines Candela’s architecture.”

For weekly email updates on residential real estate news, sign up here. Follow us on Twitter: @nytrealestate.