It's 23 October and I'm sitting in a nice house on Killiney Hill listening to David McWilliams talk about devaluing the currency while a hyperactive Labrador jumps all over me. It's a bit distracting. Then I remember that another person to be slobbered over by this dog while McWilliams talked economics was the minister for finance.

You see, the most striking chapter of McWilliams's new book, Follow the Money, a bonfire-of-the-vanities-style autopsy of the Celtic Tiger, is about a late-night visit from a weary, crumpled, garlic-chewing Brian Lenihan. It's a moment of tea, sympathy and economic wisdom, colourfully told and liberally lubricated by dog drool.

"Last September, I was asked to go on an RTé radio programme to talk about the economy," he says. "Brian Lenihan was there and the big issue was the public sector versus the private sector. Halfway through I said, 'This is all nonsense, the real problem is that the banks are going to go bust.' And everyone said, 'Oh you can't possibly say that! That's heresy!' It was like they were in Lilliput. At the end, Brian Lenihan, who I'd never met before, said jokingly, 'You should come in and advise me, sure everyone else is.' Now, I chose not to. But then I went on Prime Time and they were still saying the banks were fine. So I rang him, and then he was here. I was taken by him and liked him, but I could also see that he was isolated within the department and that he was being given bizarre options that were part of the whole denial process."

A week later, the book's version of events has upset the minister. Brian Lenihan asserts that he left the house at 12.30 not two in the morning, denies the dissident economist helped formulate the government guarantee of the banks (this was already on the cards, he insists), denies that he was unhappy with his advisers, and claims that McWilliams asked him for a job. There are also more general criticisms from various quarters that McWilliams had breached a confidence by revealing the visit.

So McWilliams spoke to me again, initially to defend his actions, asserting that he'd told Lenihan he'd be featuring the incident in his book and pointing towards a critical article the minister had written about him last April. But, a few days later a more chastened McWilliams calls to say he now wasn't sure he should have made the meeting public in the manner in which he did. "The stuff about me asking for a job is nonsense," he says. "And the facts of the evening I stand by. But on reflection, I think revealing it like that wasn't the right thing to do. I've been thinking about it a lot over the course of this horrible week [he sounds like he really means this] and I think maybe I let down people who trusted me as an independent commentator."

If that wasn't enough, it also emerged that Miriam O'Callaghan was understandably upset at his over-the-top depiction of her "flirting" with him on the set of Prime Time. (That section is a bit 'Carry On Prime Time'.) For this he gives an unequivocal apology. "I'm very sorry Miriam was upset by what I wrote," he says, sounding genuinely contrite. "I was trying to present in a light-hearted way the tension of a television studio. I never meant to suggest that she in any way used her sexuality in pursuing her career, or that it overshadowed her skill at what she does. I'm very sorry about that."

All of this is exactly the wrong type of publicity to surround a book which is all about ideas. McWilliams is a unique figure – a bestselling economist and television personality who distils economic theory into parables and caricatures and who's spent the best part of a decade predicting that bust would follow boom. His ideas, whether you agree with them or not (and I often don't), are always worth discussing.

When I met him a week earlier he was warm and funny, filling me with coffee and carrot cake made by his wife Sian, who he married in 1994 after meeting her at a friend's wedding. ("I literally scored the bridesmaid!") In conversation, like in his books, he veers towards well-defined boxes ("It's a typical Northern Irish household," he jokes, referring to his wife's cake and father-in-law's furniture. "Everything's handmade!"), has a wide breadth of reference ("You can learn as much about economics from a Thomas Hardy novel as from Paul Krugman") and seems to enjoy reaching around for unlikely metaphors.

"Economists are a bit like..." he pauses, "the Catholic church before Vatican II. All the priests speak in Latin and insist that they're the only diffusion mechanism for the message. 'Sure the plebs don't even understand the language we speak!' That's their attitude. Well, I've always seen myself as more of a Lutheran in that regard. Economics are far too important to be left to academics."

This passion for bringing theory to the masses explains the caricatures strewn throughout his work. The "Breakfast Roll Men", the "Decklanders" and in the most recent book "Fin Boys" (undereducated, overpaid young sales people with fancy hair) and "Miss Pencil Skirts" (well educated, indebted professionals facing the dole) can be very effective tools but they can also be a bit grating. Does he ever feel that by putting people in stereotyped boxes he's patronising people? "Look, the characters are there to explain how ordinary people get into very bizarre circumstances and how economics affects everyone. Two generations of economists have failed to understand that it's really only about people."

Does he have me carefully boxed into a type? "Beardy muso trying to encroach on my gig!" he blurts out with a laugh. "No I don't!" he protests, still laughing. "I just look around and observe people, and the Decklanders and Breakfast Roll Men wouldn't have had such credence if people didn't see some sort of reality in them."

One character in the current book really did ring true – the Merchant of Ennis, a developer from the midlands, incongruously pictured contemplating a failing business at the funeral of his father. McWilliams gets suddenly quiet. "The thing is, my own dad died this year," he says. "So his echo is hugely in the book. I suppose that happens to everybody when their father dies. That chapter feels real because I was at a funeral, and it was our family's funeral. Lots of things strike you at your father's funeral. I was struck by the way in which Irish people grieve and the way they come together. And you meet people who really love you and love your family and love the man who died and because this was my first close family to die, I'd never experienced it before and was touched by it."

He continues: "I was away at the time my dad died. I was in Washington. I'd no premonition of it. I was doing this TV programme and I was jetlagged so I got up at four or five in the morning and started writing about this small-time builder who'd got in over his head... and for some reason I put him at a funeral. I'd written about 1,000 words but not in detail. And my mum rang 10 hours later."

He pauses. "The Merchant of Ennis represents those characters who just went for it. They weren't devious. There was nothing cynical about them. I thought that if you put him at a funeral, you could see the love and affection of his friends and this would highlight his own duplicity when you realise he's now impoverished them." He pauses again. "Your father dying does change your whole view of the world."

This revelation makes a lot of sense. The book emotionally hinges on a period when McWilliams's father was unemployed. "My father losing his job... that wasn't supposed to happen to our family," he said. "I remember as a boy being brought to the dole office on Werburgh Street and realising that he was ashamed. That put a little burning flame of analytical rebellion into me; not physical or political rebellion, but analytical rebellion. You think to yourself – this isn't right for a grown man with his 10-year-old son. And because my son is about that age I was then, I know that that relationship is very intense – the father is a god. To see how unemployment brings good people down and depresses people and forces itself unwelcomely into families... that's a more important insight than reading great works of economic theory."

Which is why he resists the urge to proclaim "I was right!" even though, "I've spent seven years talking about what's just happened and was told by Bertie to go commit suicide. But I just had the luxury and the responsibility of publically articulating conversations that were going on in kitchens all around the country. And if you're sitting in Portlaoise and you just lost your job, the last thing you want is some twat on the television saying 'I was right!'"

Being right can be a good career move, however, and plenty of other naysaying economists have been absorbed into the system – from Alan Ahearne and Patrick Honohan, to George Lee as a TD for Fine Gael. McWilliams, however, is adamant that he doesn't want to be an insider, which is why the minister's suggestion he sought a job galls him.

"It's a bit weird, because technically I'm one of them," he says. "I came from those schools. [He went to Blackrock College.] I could be head of Ernst and Young! But I never felt comfortable in that world. It's not a world I liked. I've gone from working in the European Commission and the Central Bank, which are right at the centre of the establishment, to UBS which is the Swiss establishment, to where I am now. I presented at the Ernst and Young business awards last night so I'm not totally out of that world, but I went and played five-a-side-soccer afterwards. I'm more interested in making programmes and writing books and exposing the tightness of it all." If he was an official part of the gang he probably wouldn't observe that "Two years before, Seanie Fitz was like the pope and they were all coming up to kiss his ring."

Generally McWilliams believes the power dynamics need to change. "At the very top of the system there are deeply entrenched vested interests," he says. "When there are recessions elsewhere the insiders get blown apart. When we have recessions the insiders get stronger and that's crazy."

He resents the narrow parameters of the current economic debate. "Why the focus on the public sector? We're supposed to have a scrap with ourselves now? My mum's a teacher and my sister's a teacher – what am I supposed to do? Say, 'I don't like you'?"

And this is why he writes books and makes television shows. He thinks we should avoid the narrow focus and be more playful with our ideas. And to this end he's always willing to develop a demographic caricature or to embrace a new idea (like reaching out to the diaspora, for example, despite what I'd personally see as dodgy parochial implications). Maybe this playfulness got a bit out of hand in his depictions of Miriam O'Callaghan and Brian Lenihan. Maybe he forgot for a moment that they weren't caricatures like Miss Pencil Skirt and Breakfast Roll Man. The upshot is that this book's big idea is being overshadowed.

Because David McWilliams thinks that we should leave the monetary union (whilst strengthening Europe's political union). He makes a strong case based on the need to devalue our currency. It's an idea I'm going to let percolate before judging, but it's the one he'd no doubt prefer the country was discussing. "It's going to happen," he tells me. "We will end up doing it." And irrespective of the issues currently dominating the book's coverage, he's been right about things like this before.

Follow the Money by David McWilliams is published by Gill & Macmillan, price €16.99