They are basking sharks. They have a particular courtship display, a kind of “speed dating”. Sharks that swim in circles have a good reason to do so.
Marine biologists revealed it. They studied basking sharks seen in the west of Ireland. They engage in annual breeding behavior. It is the first place in the world where this has been verified.

Speed dating
Circular formations have been documented the last 40 years in the Atlantic off Canada and the U.S. In the coastal waters of the U.K. and Ireland they have rarely been seen. Until now, scientists could not explain the behavior.
Scientists captured images of 19 groups circling using underwater cameras and aerial drones. This was done in County Clare, Ireland, between 2016 and 2021. They found that each group consisted of between six and 23 sharks. They swim slowly at the surface, with others further down. It is a dimensional ring structure that the researchers called a “toroid”.
They were equal numbers of sexually mature males and females. Some females had a paler body color than males. The duration of the courtship toroid lasts several hours, and perhaps even several days. But individual females and males associate with most other members within a few minutes.
Concealed conduct
Professor David Sims of the University of Southampton explains in a statement. “It was a mystery. It is a kind of courtship toroid. It acts as a slow-motion ‘speed dating’ event to evaluate many potential mates at once.”
The behavior of sharks swimming in circles remained hidden for a long time. Circles are most often formed at depth away from surface observation. That would explain it.
Basking sharks are slow-moving oceanic giants. They can grow up to 12 meters long. They feed on zooplankton. Throughout much of the 20th century, in the northeast Atlantic, they were hunted for liver oil and fins. This drastically reduced the population. Basking sharks are still endangered in Europe.