The trip marks another peak on the remarkable trajectory of Bono, once just a famous musician and now known by some as The Man Who Wants to Save the World. Among Africa's leaders, he is just "Mr Bono". Declan Walsh writes from Addis Ababa.
Paul O'Neill whipped off his tie as he sat down to a poolside glass of red wine in the Ethiopian capital, Addis Ababa, last Thursday evening. The US Treasury Secretary had just been discussing how to tackle Africa's woes with two young reporters from the music station MTV - not his regular audience, he admitted readily.
"Tell me this," he said, leaning his chin on both hands. "How do we explain this stuff to a 12- or 14-year old? How do we get them interested?"
The scruffy rock star beside him had plenty of ideas about how Americans should engage with Africa, not all of them shared by his silver-haired partner. But this evening, at the conclusion of a hectic whip around Africa, Bono wanted to stress unity, not their well-publicised differences. So he joshed with the O'Neill family.
When he mistakenly described Mr O'Neill's former business as steel - it was aluminium - Mrs O'Neill perked up from the back seats. "You're dead meat now!" she shouted at the smiling pop star.
Bono and Mr O'Neill's African odyssey has been extraordinary in many respects. The man who holds America's wallet and the campaigner who wants to open it traipsed around slums together, peered down wells and inspected coffee factories and cut-flower plants.
BONO'S job was to soften up Mr O'Neill, who has famously critical views of foreign aid to the Third World. It seems to have worked, even if it's not yet clear to what degree. The Treasury Secretary was visibly touched by some of the squalor and has made clean water for Africa a personal crusade. "If you really want to change my mind about anything just give me a baby and talk to me," he said only half-jokingly on Thursday before indulging in a misty-eyed description of how he had been "adopted" by a little girl in an orphanage that morning.
He promised to spend the waking part of yesterday's 24-hour plane journey back to Washington reflecting on how to relate to President Bush the "facts and the emotions" of what he had seen.
The trip marks another peak on the remarkable trajectory of Bono, once just a famous musician and now know by some as The Man Who Wants to Save the World. Among Africa's leaders, he is just "Mr Bono".
"If I ever make a solo album - and I hope I never do - I'm going to call it 'Mr Bono To You' " he joked before leaving Africa on Thursday.
Driven first by the campaign to cancel Third World debt, now by the entire Third World development agenda, he has pulled off the remarkable coup of becoming a friend of the Bush White House, embracing even hardline Republicans such as the racist senator Jessie Helms.
The combination of celebrity and knowledge - he claims to take World Bank reports to bed these days - is the glamorous and potent new frontier in the campaigning business, once the preserve of the worthy-but-dull. But he admits that embracing the political untouchables has lost him friends and forced him to neuter his opinions.
"I've had to become apolitical to further the cause," he told The Irish Times, "and sometimes I wince."
Then again, he points out, the much-criticised proposed Farm Bill - which would sharply increase agricultural subsidies to American farmers - was put forward by Tom Daschle, a senior Democrat and prominent supporter of the debt and AIDS campaigns. The trade unions also have vociferously opposed moves allowing African goods into the US, particularly textiles.
As a recent convert to the virtues of international trade, Bono - who says he is "anti bad globalisation, not anti-globalisation" - has learned that the lines between left and right are more blurred than before. "The usual liberal allies suddenly look different," said Bono. "Now I don't want to take a position because they are all loaded."
Where does the steam for this fiery campaigning come from? Bono isn't quite sure. Swerving from theories about folk memories of the Irish Famine to something about Irish music being rooted in Africa, he suddenly halts on a conclusion: "Look, the bottom reason is not all that bollocks. It is simply unacceptable to let a whole continent go down the toilet."
Another factor is the Christian beliefs he so successfully leveraged with southern conservatives like Senator Helms. "Everyone knows that I'm a 'need to practice more' Catholic/Protestant," he said. "But I grew up with the sense that if my faith meant anything, it must be tackling inequality."
Of course, he could immediately help many poor by shelling out all or part of his vast fortune. The Sunday Times Rich List values U2 collectively at €600 million so Bono is worth at least one quarter of that. Adding Mr O'Neill's wealth - his share holdings and last year's wages alone are worth over $200 million - then the two men are worth almost as much as Ethiopia's total exports for 2000. But Bono doesn't see it like that.
"Bill Gates has the deepest pockets of anyone and he doesn't have enough money to fix the problems. There's a certain type of poverty that's structural, and we need governments to get them organised," he said.
"What we do with our personal wealth is our own business. We have to face ourselves, and I will too."
Instead, Bono's gift to the poor is his celebrity and his tireless ability to campaign in high places on their behalf. With Bob Geldof he recently set up the DATA Foundation, a high-octane lobbying vehicle that stands for Debt, Aids and Trade. Quietly supporting the two Irishmen are two other key figures - Jamie Drummond, who previously headed Drop the Debt, and Lucy Matthew, a former aid worker.
"We are getting information to America's pig roasts and backyard barbecues. Any politician knows that's where you get elected in the US," said Mr Drummond, explaining their tactics. "We've got to get Africa elected in America."
But not all are convinced that wheeling and dealing in the deep-pile carpets of power will heal Africa's woes. Campaigners such as Salih Booker of think-tank Africa Action say that Bono's value is as a shouting voice on the outside, not a whispering persuader on the inside.
Other critics say the Bono campaign - while advocating democracy and transparency - conveniently ignores many tangled political issues tied into the aid business. During some of the largest emergencies of recent years - from Biafra in the 1960s to Rwanda in the 1990s - ruthless local forces have exploited aid as a tool of war. Bono's focus on funding - or Mr O'Neill's obsession with results - may detract from a wider debate on how aid can harm as well as help.
But as he returned home yesterday, Bono could have a sense of a job well done. Mr O'Neill was undoubtedly affected by his African tour. Whether that will translate into a policy sea change in the world's richest country - but stingiest aid donor - remains to be seen, but the omens are positive.
And, amid the goodwill, some things never change. On their last night, Mr and Mrs O'Neill tucked into bed before the long journey ahead. But Bono and the Hollywood actor Chris Tucker headed for the hotel nightclub, where the two stars captivated the dancefloor. Bono may be a self-admitted "policy wonk" now but at least he hasn't forgotten that it started with rock'n'roll.