The ambush was simple. A spotter on a hill would scan the sea and when he saw the big black fins approach, he would shout down to the boatmen. They would ready their nets and quickly row out to the kill zone.

When a shark got tangled in the mesh, Brian McNeill would wait a minute or two while it struggled, then steady himself and raise his harpoon. This was the crucial moment. The creature would be diving and thrashing, desperate to escape. If the blade hit the gills blood would spurt, clouding the water. The trick was to hit a small spot between the vertebrae.

“It was very hard to get him exactly in the vertebrae,” McNeill recalls. “He’d be spinning and diving all the time. If you happened to get the spear in that inch and a half, that was him. His eyes would roll back in his head and he wouldn’t move again.”

And so another basking shark would die off Achill, a County Mayo island on Ireland’s Atlantic coast that was a gathering point – and then a graveyard – for a mysterious, majestic species.

The slaughter in Keem Bay began in the 1950s when islanders discovered there was money in the livers and fins of the plankton-eating visitors, which arrived in spring and left in summer.

Crews in currachs, traditional wooden boats, greeted them with nets and harpoons. Some seasons more than 1,500 carcasses littered the island’s beaches and jetties.

Over the years the visitors became fewer and the catch dwindled to a few hundred, then a few dozen and by the 1980s just a handful every season.

In 1984 only five were caught, after which hunting was abandoned. McNeill was part of the crew that caught the last shark. After that, sightings became very rare.

Whatever sharks were left, they seemed to forsake Achill. “The sharks disappeared,” says McNeill.

Now aged 76, he is one of the last living connections to the hunting era – and a witness to an unexpected coda: the sharks are back.

Sightings of Cetorhinus maximus, the world’s second-biggest fish after whale sharks, have proliferated in Achill and other parts of the coastline of Ireland and Britain in recent years.

The phenomenon has intrigued and delighted researchers, given that the species is classed internationally as endangered and facing a high risk of extinction, according to the Irish government.

“It’s astounding,” says Alex McInturf, a coordinator for the Irish Basking Shark Group, an international team of scientists. While sightings in New Zealand and the north-west US have declined, they have surged in Ireland and Scotland.

“They are one of the only locations where you can see basking sharks regularly and in large numbers,” she says.

“The crazy part is that the fishery in Ireland used to be the biggest in the world yet Ireland seems to be the place where they have recovered the most. You would have expected the opposite.”

Despite their size – up to 10 metres long – the sharks are hard to count and track, says McInturf, a postdoctoral fellow at Oregon State University’s Big Fish Lab and a visiting researcher at Queen’s University Belfast.

In some seasons, dozens are spotted; in other years its hundreds, she says. “We don’t know what’s driving those variations, we’re trying to figure that out.”

One theory concerns connectivity between Irish and Scottish waters. Sharks have been filmed circling each other in tight formations, prompting speculation of courtship rituals.

In 2022 Ireland extended legal protection by making it an offence to hunt, injure or wilfully interfere with basking sharks’ breeding or resting places, a move long sought by campaigners. The UK has similar legislation.

McNeil blames the visitors’ long absence on offshore salmon trawlers inadvertently snagging the sharks in drift nets, a practice that was made illegal in 2007.

Whatever has brought the creatures back, McNeill shares the delight of islanders and tourists, who flock to beaches to savour the spectacle. “I’m happy they’ve made a comeback,” he says. He does not, however, regret hunting them.

Originally from County Monaghan, McNeill worked as a chef in London before moving with his wife to Achill in 1971. “I just got tired of the tube and the crowds.”

He joined a four-strong team on a currach that fished salmon and sharks. “They were such enormous fish and they were really quiet until they went into the net.” Sometimes they escaped the trap. “If they were big and strong they could bust through the net, no problem.”

He estimates he killed up to 300, using harpoons they made from a car chassis, but admits he felt some sympathy for them. “They were doing no harm to anybody,” he says.

But he had no compunction during a hunt, when a boat risked being overturned by the huge fish. “You’re only thinking about getting rid of the danger as quickly as possible.

“I was happy that they were dead so we were out of harm’s way,” he says. “It was self-preservation.”

Livers were turned into oil and the fins sold as delicacies to countries in Asia, yielding income that let fishers and their families on the borderline of poverty stay on Achill rather than emigrate, says McNeill. “Everybody had money and it was a good life.”

He looks forward to calm, sunny days, ideal conditions for shark-spotting from his home in Dooagh village, which overlooks another bay.

“It’s lovely to see them. I’m just happy that we’re not killing them any more.”