Limerick brothers and highly gifted students John (27) and Patrick (29) Collison each own around 12pc of their online payments business Stripe, making them the world's youngest self-made billionaires. They each have a net worth of about $1.1bn after it was valued at $9.2bn in 2016. It may well have increased in value since on the back of a deal with Amazon last August that may be significant, though neither firm commented. Other big online names - including Chinese giants AliPay and WeChat - are among its thousands of customers.
Now employing over 750 people, Stripe has shaken up the sector, by simplifying online payments in the face of the likes of PayPal, while also eating the traditional banks' lunch. It has also gone into other niches, making it easier for online sellers to register a US company and bank account, and acquiring Payable, which simplifies doing taxes and paying freelance and contract staff, for example. The brothers have been making money since their teens - they sold their first company, Auctomatic, for $5m in 2008.
They both secured highly sought-after places in Harvard and MIT but dropped out to pursue their entrepreneurial leanings and set up Stripe in 2010. With initial backing from Tesla founder Elon Musk and PayPal's Peter Thiel, the company has become an exemplary case study in how to scale a tech start-up.
The two now back start-ups themselves as well. Stripe put money into UK banking app Monzo last year, while other investments have been in fellow fast-growing tech firms Intercom, Slack, and a number of less well-known outfits including Exec, Retool and Cruise Automation. The latter sold for $1bn in 2016 to General Motors having taken only $18m of investment, and the Collisons invested in a $4.2m seed round in 2014.
Patrick revealed in an interview last year that he keeps a clock on his desk that counts down how long it's estimated he has to live - as a reminder not to waste time. The two, who are both single, are voracious readers, with stacks of books by their beds and adorning bookshelves in their modest apartment. They hike, run and cycle; and also own their own small plane, a four-seater, twin-prop Diamond D42. The fall in their wealth this year is attributed to the euro being stronger against the dollar by about 10pc, while other investments and assets are conservatively estimated at €60m.