It’s one of the most notorious stories of the late 20th century: the 1975 disappearance and presumed murder of Jimmy Hoffa, once the most powerful union boss on Earth.

Now, intrigue in Hoffa’s mysterious death is stronger than ever thanks to the Golden Globe-nominated Martin Scorsese film, “The Irishman.” While the film follows many real-life events, some dispute the accuracy of the three-and-a-half-hour saga, which is based on a book.

Al Pacino plays Hoffa in the film, which delves into the corrupt union boss’ rise to fame and disappearance — the subject of speculation for decades. Frank “The Irishman” Sheeran (played by Robert De Niro) claimed to have been the killer in a book written by Sheeran’s lawyer, Charles Brandt, “I Heard You Paint Houses” — mobster code for “I heard you kill people.”

While Hoffa’s death is still a mystery, Sheeran claims in the book that he allegedly shot his longtime friend Hoffa in the head after luring him to a house in Detroit on orders.

The book was published in 2004, the year after Sheeran died and nearly 29 years after the murder was committed. While it was too late for law enforcers to do much — Brandt said that they “dug up floorboards [of the murder site in 2013] for analysis and found human blood but could not tie it to one person” — Sheeran had once occupied a place on the FBI’s shortlist of possible suspects.

His confession to killing a man whom he called a friend illustrates the hard choices that come with a life dedicated to crime. “Frank whacked guys,” Brandt said. “I estimate that he killed 25 to 30 people. He learned right away that you don’t say no.”

Unlike a lot of men who wind up killers for the Mafia, the real-life Sheeran born in Darby, Pa. had no criminal connections. After joining the military in 1941, Sheeran was sent to Italy where he developed a knack for killing — a skill that would come in handy off the battlefield.

”His lieutenant told him that when you are commanded to ‘interrogate somebody and hurry back,’ you are going to kill the guy,” said Brandt.

In 1945, Sheeran moved to Philadelphia, where he started a family and got a job as a truck driver for a grocery chain. His first arrest came two years later when he was charged with disorderly conduct after beating up two men in an altercation on a trolley.

In 1955, he had a chance meeting with Russell Bufalino, boss of the northeastern Pennsylvania crime family. In short order, he began doing tasks for the Kingston-based Bufalino, like chauffeuring him and making deliveries.

Coincidentally, this was around when Sheeran was making extra dough by collecting money for small-time Philly loan sharks. Seduced by the lifestyle, Sheeran said yes when a local mobster called Whispers offered him $10,000 to burn down the office of Cadillac Linen Service, which was competing with a company that Whispers had an interest in.

But Sheeran was spotted while scoping out the place — and it turned out that Cadillac was owned by a friend of Bufalino’s. The moment plays out in “The Irishman” much like Brandt tells it, but the scene adds Harvey Keitel as boss of the Philadelphia crime family, Angelo Bruno.

“Because Frank had been seen in [Bufalino’s] company, the friend did not have Frank killed,” said Brandt. “But Frank was told to make it right by killing Whispers. That night was his first hit.”

In 1957, as a reward for pulling off the job, Bufalino introduced Sheeran to Jimmy Hoffa, the president of the International Brotherhood of Teamsters labor union notorious for Mafia ties, corruption and violence.

Hoffa needed somebody with muscle to silence enemies. He told Sheeran over the phone, “I heard you paint houses,” a euphemism for blood splatter on a wall. Sheeran said he replied yes and added, “I also do my own plumbing” — meaning, he disposed of the bodies too.

Sheeran grew close to Hoffa and eventually got a lucrative union-boss job as president of the Local 326 in Wilmington, Del., which had him raking in under-the-table rewards for mob favors. More notably, he served as Hoffa’s muscle: beating up enemies, killing people trying to start rival unions and running guns.

In the book, Sheeran also claimed to have transported rifles from Brooklyn to Florida for the killing of President John F. Kennedy, adding credence to theories that Hoffa and the mob played a role in JFK’s assassination. The president, along with his brother Robert F. Kennedy, had a strong disdain for the union corruption that Hoffa stirred up as well as the mob.

However, Scorsese ended up leaving the JFK subplot on the cutting room floor and instead summed up the conspiracy confession from Sheeran in one subtle blink-or-miss-it line: When Pesci’s character says, “If they can knock off a president, they can knock off the president of a union.”

“That’s the only one I allowed in, because you can interpret that, if you want, meaning ‘they’ knocked him off, we didn’t knock him off, but people can be taken out,” Scorsese said, according to Indiewire.

Per the book, one of Sheeran’s biggest hits happened in Little Italy on April 7, 1972: the murder of Colombo family mobster Joey “Crazy Joe” Gallo, at the behest of Bufalino.

It was known that Gallo would be celebrating his birthday at Umberto’s Clam House on Mulberry Street. Sheeran said that he walked inside, dressed casually, posing as a truck driver who needed to use the bathroom.

Although startled by the presence of a woman and little girl at the table where Gallo and his crew sat, Sheeran had his marching orders and began shooting. Gallo headed for the door, making it outside before being taken down by three bullets. Sheeran escaped in a waiting car.

The year that Gallo was murdered, Hoffa, who went to prison on racketeering charges in 1967, was eager to regain control of the International Teamsters. But Mafia kingpins didn’t want him back.

Heat on the persistent Hoffa went on. He started getting hang-up calls and bullets fired through his window at union headquarters and somebody even blew up his 45-foot cruiser, which was docked in the Detroit River.

When Hoffa still refused to acquiesce, Brandt wrote that the mob turned to the one man who could lure him to a vulnerable location.

That’s where many say the film and the book divert from reality.

In late July 1975, Sheeran flew from Ohio to Pontiac, Mich., to murder his mentor, Brandt wrote, adding that he “felt nothing.”

“Frank could not blink, much less say no [to killing Hoffa],” said Brandt. “Or else . . . they both would have gotten killed.”

According to Brandt’s account, Sheeran drove with a few other associates to pick up Hoffa at a restaurant called the Red Fox. Sheeran claimed that his presence helped put Hoffa at ease about driving to a meeting at a Detroit house.

They arrived and entered the vestibule of a home that was obviously empty. “When Jimmy saw . . . that nobody came out of the rooms to greet him, he knew right away what it was,” Sheeran said in the book, adding that Hoffa tried to flee. “Jimmy Hoffa got shot twice at a decent range — not too close or the [blood] splatters back at you — in the back of his head . . . My friend did not suffer.”

Soon after, Sheeran claimed, Hoffa’s body was turned to ash at a crematorium.

But not everyone buys his story. Dan Moldea, author of the deeply researched “The Hoffa Wars,” insists that Sheeran did not kill Hoffa.

Moldea — who interviewed mob figures, investigators and prosecutors for his book — agrees that Sheeran flew to Pontiac and lured Hoffa into the car. But he believes that the murder was committed by Salvatore “Sally Bugs” Briguglio, an enforcer for the Genovese crime family. Moldea bases this on interviews with parties including the owner of a New Jersey dump where some believed Hoffa’s body was disposed.

“This is a one-source story about a pathological liar,” Moldea told The Post of Brandt’s book on Sheeran.

He voiced his displeasure about the De Niro-starring flick when he met the actor at a dinner in 2014.

“De Niro had a lot of pride that he is doing the real story,” said Moldea. “I told him that he’s been conned.”

But Brandt sticks by his story. After Sheeran served 13 years of a 32-year prison sentence for labor racketeering and was crippled by arthritis while living in a nursing home, he confessed to killing Hoffa to three priests as well as to Brandt.

“Frank sought forgiveness and, to his way of thinking, died in a state of grace,” said Brandt. (He alleged that Sheeran committed suicide, in 2003, at age 83, by starving himself to death for six weeks.)

Whatever the facts are, De Niro, Scorsese and screenwriter Steven Zaillian are going for Brandt’s version of it.