The Irish tech entrepreneur was an angel investor in the call-a-cab app that raised €22m funding last week from Richard Branson and Japanese and Russian backers and is now worth €104m.
A chunk of the fresh finance will go to its ambitious European expansion plans, with €1.1m being spent on each new city launch.
"I got involved as an angel then decided to join the company," said O'Cuilleanain, who is the company's Europe, Middle East and Africa boss. "I've known Ron [Hailo's chairman] for 20 years. He phoned me up knowing I was involved in tech start-ups and after five minutes I said I thought it was a smashing idea and was willing to get involved."
O'Cuilleanain was one of a batch of early investors and small venture capitalists who put €1m into Hailo, with Skype founder Niklas Zennstrom adding €1.5m. His five-figure investment will have soared in value.
O'Cuilleanain became a millionaire with the sale of an outdoor ad business a decade ago and sold a company called Netsure to Oracle in 2007.
Hailo is setting up in Madrid and Barcelona, with rapid expansion planned into other cities including Paris and Amsterdam. Cork and Galway will be added in the next six weeks.