They've been dubbed the odd couple – one a publicity-shy techno geek who is the second richest man in the world; the other an ageing rock star who wants to change it. But Bono and Bill Gates's shared plan for the planet has forged such a bond between them that they're fast becoming best friends.
This week, U2's frontman welcomed the Microsoft tycoon to Dublin as part of their ongoing tour to shore up support for foreign aid, and convince European leaders that even in tough times wealthier countries have a moral obligation to provide financial help to ones that are poor.
Gates met Tánaiste and Foreign Affairs Minister Eamon Gilmore to discuss overseas aid, the EU budget and the campaign to end polio. He also met the Taoiseach and President Higgins as part of a visit to a number of European countries ahead of the World Economic Forum in Davos, Switzerland.
Growing scepticism among recession-hit western powers and economists about whether aid actually eases poverty or too often ends up in the wrong hands, has strengthened the friendship between Gates and Bono and made them critically aware of the benefits of presenting a joint front in public.
They know the cameras can't resist the sight of them together on the international stage shaking hands with presidents and policy-makers, and that ensures their agenda makes the headlines.
But it is the depth of their relationship away from the limelight which is even more interesting. Their shared sense of impatient optimism and messianic drive to change the world has given them a common purpose, but, in the process, they have discovered other similarities in everything from religion to parenting.
When Bono is on the West Coast of America, he stays at Gates's home, a $150m lakeside mansion in Washington state, which features a swimming pool with underwater sound system, a trampoline room, and a library which holds Bill's pride and joy – the Codex Leicester, one of Leonardo da Vinci's notebooks, which he bought in 1994 for $30.8m.
Not so long ago, there were rumours that the pair were planning to buy a 65-acre island off the coast of Mayo, where they could hammer out ideas and rekindle their long-distance friendship.
But their friendship didn't start out so well.
When the singer first tried to recruit Gates to his poverty crusade more than a decade ago, the socially awkward software guru wasn't exactly pro-Bono in his response. In fact, he dismissed the idea.
Nancy Gibbs, a reporter with Time magazine, described how "the nature of Bono's fame is that just about everyone in the world wants to meet him – except for the richest man in the world, who thought it would be a waste of time".
"World health is immensely complicated," said Gates, recalling that first encounter in 2002. "It doesn't really boil down to a 'Let's be nice' analysis. So I thought a meeting wouldn't be all that valuable."
But he had a sudden change of heart when he did eventually accept the invitation.
"It took about three minutes with Bono for Gates to change his mind," Gibbs said. "Bill and his wife Melinda, another computer nerd turned poverty warrior, love facts and data with a tenderness most people reserve for their children, and Bono was hurling metrics across the table as fast as they could keep up.
"He was every bit the geek that we are," said Gates Foundation chief Patty Stonesifer, who helped broker that first summit. "He just happens to be a geek who is a fantastic musician."
Since then, their unlikely alliance has grown closer, verging on the sycophantic at times. During concerts, Bono has taken to celebrating Gates on stage, bursting into a round of 'Happy Birthday' for him on one occasion, and describing how meeting him was a 'life-changing experience' on others.
The Microsoft billionaire, who stepped down from the company four years ago to become a full-time philanthropist, is equally gushing about his pop-star pal.
"I get to hang out with a lot of very cool and inspiring people, but none more so than Bono. I first met him a decade ago, and we've worked together a lot since. But people are still curious about our connection. I guess we might seem like a strange pair. We are. Melinda and I have many friends, allies and valued partners in our philanthropic work, but few as creative, energetic and inspiring as Bono."
In 2005, the trio were pictured on the cover of Time magazine when they were voted People of the Year for their humanitarian work. Dubbed "The Good Samaritans", Bono picked up the title for "charming, bullying and morally blackmailing the leaders of the world's richest countries into forgiving $40bn in debt owed by the poorest"; Bill and Melinda Gates for "building the world's biggest charity and giving more money away faster than anyone else ever has". The latest figures show that he has so far given away $28bn via his charitable foundation.
Having Gates beside him on the global platform has given Bono gravitas at a time when he is seen by some critics as a faux expert who should keep his nose out of the world economy.
He has spoken scathingly of Bono, describing him as a "ridiculous celebrity wonk" who does not challenge power but embraces it.
"He is more likely to appear in photo ops with international political leaders," he says, "or to travel through Africa with a treasury secretary than he is to call them out in a meaningful way."
Easterly's iconoclastic book The White Man's Burden argues that the $2.3 trillion of foreign aid spent by the West in the last five decades has failed to lift the poorest countries of the world out of poverty or stimulate economic growth.
Gates said he hated the book, sniping back that success can be measured in things beyond economics, such as literacy and life-saving vaccines.
Another critic of celebrity philanthropy is Zambian economist Dambisa Moyo who believes it is wrong that famous westerners have become "inadvertently or manipulatively spokespeople for the African continent".
The Harvard academic believes foreign aid has largely resulted in holding Africa back.
"You get the corruption. Historically, leaders have stolen the money without penalty, and you get the dependency, which kills entrepreneurship," she says.
Despite the critics, Bono is convinced that Gates has the power and determination to bring about real change. "Bill wants to know where every penny goes," he said.
"Not because those pennies mean so much to him, but because he's demanding efficiency.
"When an Irish rock star starts talking about it, people go, yeah, you're paid to be indulged and have these ideas. But when Bill Gates says you can fix malaria in 10 years, they know he's done a few spreadsheets."
As both men approach 60, and a dawning realisation that they are mortal, they have become more vocal about their spiritual beliefs. While fans of Bono are well used to him quoting passages from scripture and wearing his Christianity on his sleeve, Gates, who typically avoids talk of religion in public, said this week that he no longer had any need for money and that he was now doing the "work of God".
But as fathers of teenagers, they are still busily engaged in the business of parenthood. too, and share common tactics to try and shelter their children from the mindboggling wealth in their lives.
Bono's 21-year-old daughter Eve said recently that she was banned from her parent's multi-million penthouse in Manhattan when she was studying there, and had to share a small apartment with a friend instead.
Gates has vowed his three children will inherit no more than a slither of his $56bn fortune – "enough to let them do anything but not enough to let them do nothing".
His plan is to have most of it spent before he dies, an ambition his new best friend is only too happy to help him fulfil.