For two young founders of a billion-dollar start-up, the Collison brothers are remarkably self-effacing. Patrick and John grew up in a small hamlet outside the Irish city of Limerick in the 1990s and now live in a modest two-bedroom apartment in San Francisco’s fashionable Mission neighbourhood.

Along the way, they dropped out of Harvard (John) and MIT (Patrick), founded the start-up Auctomatic, sold it for $5m, and then in 2010 founded their current venture, Stripe, an online payments processing company recently valued at $1.75bn.

John, 23, stands in front of a living room wall against which dozens of books are stacked, and welcomes me to the apartment the brothers moved into about a year ago. “The caveat with all this is Patrick and I have not finished furnishing this place properly, which is why there’s that bag of random stuff and an Ikea desk and a bunch of fold-out chairs and what is actually outdoor furniture that we’ve been sitting on inside, and so I apologise for?.?.?.?”