“Vanilla,” young Connolly replied.

Bulger was convicted of a series of bank robberies in 1956 and sentenced to 20 years in jail. He was considered so dangerous that he spent time in the notorious Alcatraz prison in San Francisco Bay.

Meanwhile, John Connolly went to university and graduated from law school, before joining the Federal Bureau of Investigation (FBI). In 1975, he was posted to Boston, where he recruited Whitey Bulger as an FBI informer.

It was a mutually beneficial arrangement. Bulger provided information leading to the arrest of criminals. Once they were out of the way, he moved in on their patch.

Howie Winter, the leader of Boston’s Winter Hill gang, and 21 of his colleagues were indicted in 1979 for race-fixing. Bulger and his sidekick, Stephen Flemmi, were also involved, but Connolly persuaded the FBI not to go after them. Bulger then took over the Winter Hill gang.

In 1980, Bulger and Flemmi planted a surveillance bug in the headquarters of local mafia boss Gennaro (Jerry) Angiulo, whose Patriarca crime family controlled racketeering throughout Massachusetts. Angiulo, his lawyer, and 22 other mobsters were indicted, allowing Bulger and the Winter Hill gang to take over the racketeering operation.

Joe Murray of Charlestown, Massachusetts, a big-time drug smuggler, suffered a crippling blow when — as a result of information from Bulger — the FBI and Drug Enforcement Agency (DEA) raided Murray’s main warehouse full of cannabis in 1982.

While the FBI may have thought it was using Bulger, it could easily be seen the other way around. Bulger was not leading a fight against drugs; he was using the FBI to take over the drugs scene in Massachusetts by supplying the tip-offs that would drive his rivals out of business.

Murray was later involved in arranging to buy and smuggle guns to the IRA. In January 1984, he visited Amsterdam and Ireland, along with one of his men, John McIntyre, to arrange the shipment.

The guns were to be transported across the Atlantic in an ocean-going fishing vessel, the Valhalla, which set sail from Massachusetts. The plan was to rendezvous with the Marita Ann in international waters off the Kerry coast in September 1984.

The Valhalla was carrying $1 million worth of guns and ammunition. It was believed that the money to buy the weapons was collected by Murray and Bulger.

Bulger tipped the FBI off to the Valhalla mission. Providing such information enhanced his standing as an informant.

Seán O’Callaghan, the garda informant within the Provisional IRA, warned the Special Branch that the Marita Ann was going to be used. Hence it was kept under electronic surveillance from the Irish side, while the Valhalla was tracked across the Atlantic by satellite.

The guns were transferred at sea, with virtually the whole Irish navy waiting off the Kerry coast. The Marita Ann was seized making its way back to port. Martin Ferris, now Sinn Féin TD for Kerry North–West Limerick, was among several arrested.

While Bulger was informing for the FBI, other agencies had him under surveillance, and the home of one of his two lovers was bugged. When the news of the seizure of the Marita Ann was broadcast on television, he was recorded as saying: “That’s our stuff.”

After the Valhalla returned to Massachusetts, the crew went into hiding. The 32-year-old McIntyre offered to co-operate with police in a desperate effort to extricate himself. He was clearly “petrified” of his colleagues, said Detective Richard Bergeron, and his information proved vital.

“You don’t know where you’re going to end up or what kind of demise you’re going to come to,” McIntyre said. “I just sometimes feel like I’m trapped in this whirlpool and I can’t get out of it.”

In the early 1980s, he got involved in Murray’s drug-smuggling. He sailed on seven boats that smuggled marijuana into Boston.

“I was the engineer on all those boats,” McIntyre said. Each carried “between two and three thousand bales”. He gave up details of a drug shipment that was due to come in on the freighter, Ramsland. The boat was searched as it entered Boston Harbour on November 16, 1984, and a shipment of 36 tons of marijuana was seized.

DEA investigators were almost euphoric about McIntyre’s co-operation. “This guy had a mountain of information,” Bergeron later explained.

The DEA shared the information with the FBI. Agent Roderick Kennedy of the FBI’s Boston office interviewed McIntyre, and reported that McIntyre talked at length about Joe Murray, as well “Patrick Nee of South Boston, Kevin, and an individual named ‘Whitey’ who operates a liquor store”. This was obviously a reference to Bulger, whose business front was the South Boston Liquor Mart. The Kevin mentioned was apparently Kevin Weeks, who was also known as Bulger’s surrogate son.

Two days later, McIntyre disappeared. His body was not found until January 2000. Murray was convicted of drug-smuggling. He served his sentence and was, shortly afterwards, shot dead by his wife in a domestic argument.

When Connolly retired from the FBI in 1990 after 22 years of service, he was one of the force’s most celebrated agents. Without Connolly to protect them, however, Bulger and Flemmi soon came under scrutiny.

In 1995, Flemmi was arrested. Weeks testified that Connolly had tipped him off to warn Bugler of his impending arrest. Bulger went on the run. The Winter Hill gang disintegrated, and its members began singing like the proverbial canaries.

Connolly was convicted of racketeering in 2000 and jailed for up to 20 years. In 2006, Flemmi testified that McIntyre was killed after Connolly let it be known that one of the people who had been on board the Valhalla was co-operating with authorities.

In 2008, Connolly was convicted of second-degree murder in connection with one of Bulger’s hits, and is likely to spend the rest of his life in jail.