Visitors to the Abbey Leix estate in County Laois, Ireland, must drive on an unkempt gravel road that leads through a thick forest and past a derelict corn mill to get to the giant old house in the country’s midlands. Stephen Kinsella, an Irish economist, made the journey last May at the invitation of a friend who had recently purchased the 1,120-acre estate for more than $15 million, with plans for extensive renovations. Kinsella asked his friend why he bought the property.

"It needs someone like me," said John Collison, the 32-year-old co-founder and president of Stripe, who, with his brother Patrick, has become one-half of the most famous entrepreneurial duo in their native Ireland. The manor—which reportedly contains the nation’s oldest oak tree as well as a stud farm, walled gardens and a clock tower—was the most high-profile in a string of recent extracurricular investments by Collison. It was also a bit of a narrative violation for the brothers Collison.

For years, the tech media had characterized John and Patrick, 34, as boyish business prodigies who monkishly shared an apartment, rode rented e-bikes to work, and preferred reading economic histories and science fiction to ostentatious displays of their growing wealth. John, a student of capitalism who encourages employees to read biographies about moguls like Larry Ellison and John Malone, has lately come into his own as a brass-knuckled manager who has assumed critical leadership roles as his company hurtles toward its next iteration. At the same time, he has been adding a new chapter to his own billionaire’s journey, with a much-loved phrase serving as an epigraph: "The world is a museum of passion projects."