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.
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.