Patrick and John Collison were walking home from dinner one night in October 2009, talking yet again about their latest idea for an online payments company, when John stopped and turned to his brother.

"Why don't we just go build it?" John said, as Patrick later recalled. "It probably won't be all that hard."

Never mind that the payments industry was notoriously complex and already home to major, established tech companies like PayPal. Never mind that Patrick was just 21 and John was still a teenager. It probably won't be all that hard.

The most shocking thing about John's statement, though, was that in some sense it proved to be true. Barely five years after that conversation, their startup, Stripe, has emerged as a leader in the payments space, with a multi-billion dollar valuation and a fast-growing list of influential partners.

When Apple unveiled its long-awaited mobile payments service this year, it was Stripe — not PayPal or Square — that earned a spot on the coveted list of initial partners. When Twitter and Facebook decided to embrace ecommerce with the launch of "Buy" buttons, they turned to Stripe to help process the payments on site.

If you pay for a ride with Lyft, order groceries to be delivered by Instacart or purchase cleaning services through Homejoy then you're using Stripe as well — even if you don't know the startup's name. These and other businesses can embed a few lines of code from Stripe, as simply as one might embed code for a video or a tweet, and then begin processing credit cards as seamlessly as Amazon. Stripe handles everything else: fraud prevention, currency conversions, you name it. In exchange, Stripe takes a 2.9% cut plus a $0.30 charge per transaction.

The goal, according to those close to Stripe, is to expand Internet commerce around the world and help the next generation of businesses quickly get set up to accept payments without having to hire dozens of developers and billing experts. This proved to be a breakthrough year for Stripe, but it was several years in the making.

From the middle of nowhere in Ireland to Silicon Valley

Stripe's founders grew up in the rural town of Nenagh in County Tipperary, Ireland.

The brothers trying to reimagine Internet commerce spent much of their childhood without Internet. At first, Patrick didn't have access to a computer at home so he resorted to reading about the Internet in library books. When he and his brother did get to use a computer, the Internet on it was painfully slow in their rural town of Nenagh, Ireland.

Despite their connectivity issues, Patrick and John grew up in a technical household. Their father was an electrical engineer and their mother was a microbiologist who ran a quality assurance business out of their house. Patrick tinkered on her business computers, taught himself how to code and won an award for his programming work at the tender age of 16. John, two years younger, was whip-smart too, albeit somewhat less technically minded than his brother.

Their parents were supportive enough to let Patrick move to Boston to attend MIT when he was 17 and open-minded enough not to stop him from leaving school after one term to try his luck with startups. Patrick decided to take on eBay by building an online auction company called Shuppa, with John, who was then 16 and taking a gap year in high school.

"In terms of being cofounders, it's worked out very nicely," Patrick told Mashable in an interview in January about the experience of working with his younger brother. "Fast-growing companies tend to have cofounder strife that develops. Luckily myself and John had all our battles out of the way when we were young."

Soon enough, the brothers shifted gears and decided to sign on as cofounders for a similar startup in the works called Auctomatic. In 2008, it was acquired by a Vancouver-based domain name company for $5 million in 2008. Patrick was still a few months shy of turning 20. He moved to Canada to work at his new parent company, but after about a year, Patrick decided to return to MIT.

"The reputation Patrick had on campus was that he was a brilliant guy. He had already built a small company," says Hemant Taneja, who taught at MIT at the time and is now managing director at General Catalyst, which has invested in Stripe. "I went to MIT as well and I was thinking, 'What did I do when I was 19?' That was my initial impression. Patrick is a rare, rare individual. Intellect, execution, capability."

John had already started school at Harvard and the brothers resumed working together on side projects. In the process, they ran up against the difficulty of accepting payments and decided to work on a fix for themselves first, before realizing they weren't alone.

"We realized the problem was much larger," Patrick told me in our earlier interview. "It wasn't just a problem for individual developers like us."

So the brothers decided to solve it.

The rise of Stripe

Elad Gil heard about Stripe when the product was still in alpha and immediately recognized the need for it. "I wished it was around when my startup was up and running," says Gil, whose startup Mixer Labs was acquired by Twitter in late 2009. Gil reached out to Patrick and became an early investor in the company.

That experience, in a nutshell, is what helped Stripe stand out from the pack of payments service: it was geared very much for startups and developers.

"Payments themselves are not that important. It's the last thing you think about and can be kind of a headache," says James Wester, a research director with IDC. "What Stripe did was simplify that process and really cater to developers. It said, 'You go develop you application and we'll take care of the payments.'"

Taneja, the Stripe investor, remembers asking Patrick in the early days who Stripe's biggest customer would be. "He said, 'They probably haven't been born yet.' His whole point is there are so many new companies getting created and only 2% of commerce was online."

Perhaps the best example of that thinking is Lyft, a popular and well-funded ride-hailing service founded in 2012. The car and mustache may be the face of the company, but it's the seamless and automatic transaction experience that allows Lyft to function and that runs on Stripe.

"When you think about companies like Lyft that probably wouldn't have been able to exist five years ago because it would have been too hard to cut checks to drivers," says Cristina Cordova, who heads business development and partnerships at Stripe. "Those are companies that we're really interested in exploring brand new business models with."

Stripe is a key reason that some of these on-demand startups have been able to get off the ground so quickly — and Stripe has prospered in return. Tens of thousands of businesses now use the Stripe API for payments. Stripe has raised nearly $200 million in funding and it has brought on more than 100 talented employees. (Patrick, insiders say, has a knack for attracting smart and entrepreneurial engineers.)

The breakthrough year

For Stripe, the year started off with the news that it had raised $80 million in funding at an eye-popping valuation of $1.75 billion and ended with the news that it had raised another $70 million at double that valuation. In between, the startup scored a number of high-profile partnerships including with Apple, Twitter, Alipay and others.

Suddenly, Stripe's notoriety started to approach that of the many companies it has helped power behind the scenes.

"Obviously we were much less well-known previously. I think funding and some of the other partnerships we've launched recently have really changed that for us," says Cordova, Stripe's business development exec.

Funding and partnerships weren't the only reasons for Stripe's momentum this year. Investors point to other events like Braintree, a Stripe competitor, getting acquired by eBay late last year. Stripe used some of its funding to invest in international efforts; the service is now available in 18 countries. It also continued to build out its suite of supplementary services, including launching a tool that lets businesses charge customers in whatever currency they're comfortable with.

"Everyone is kind of focused on the fact that we do payments and basing a lot of their assumptions about our business on that," Cordova says. "In reality, we are building the products and tools that help businesses run efficiently... without needing a 50 person tax and compliance team."

That is what Stripe will be working to solve globally in 2015 and beyond.

"We don't really see anybody else approaching the problem the way that we do and thinking about building the platform of Internet commerce," Patrick told me in our earlier interview. "We want to increase the GDP of the Internet."