It's a tale of two Irises. There is Iris Robinson, dutiful wife and committed Christian, quoting scripture and denouncing the sexual immorality of others. And then there is Iris the woman who walks on the wild side, takes a teenage lover, and craves the forbidden fruit.

In all the years I've known her, there was always this internal struggle. Sometimes, she'd say she liked nothing better than jumping into her Mini, speeding along the motorway, blasting Bruce Springsteen.

Other days, she'd insist she no longer listened to rock, it was unchristian. Gospel music was all she played. Her favourite book was the bible, she'd say. She read it every day "for correction and direction".

Next minute, she'd enthuse about her favourite film, Gone With The Wind, and its tempestuous heroine. "Scarlett O'Hara is wonderful. What a spirit!" she'd declare. At home, Iris's ornaments reflected her complex personality. Marie Antoinette sat inches from the Last Supper.

In her authorised biography, Robinson said as a child she'd loved hearing stories of Jesus: "I knew I was supposed to be a good girl. I did try for a few days, then I was as bad as ever!"

The grown-up Iris faced similar turmoil. "I would love to have Moses, Jacob and Christ around a [dinner] table to hear what they have to say," she announced, predicting she'd meet them in the next world.

Yet in this world, she was enjoying not so spiritual pleasures with Kirk McCambley, her handsome 19-year-old lover. He was young enough to be her grandson. Iris's eldest son, Jonathan, is in his late 30s.

From a feminist perspective, perhaps Robinson's affair with her toyboy is to be savoured. How many aging men enjoy young sexy girlfriends without anyone batting an eyelid? And as a woman of pensionable age, her libido is surely to be celebrated.

Iris never did anything without passion. I always found her a warm, kind, emotional woman. But there is something highly disturbing about the affair. She knew McCambley from when he was nine years old and working in his father's butcher shop. She was a middle-aged woman then.

For her later to flash £50,000 in cash before the boy she says she looked upon as a son, to set him up in business, and then to begin an affair when he was extremely vulnerable – he was an only child whose father had just died from cancer – reeks of exploitation.

She was highly manipulative too. She secured the money for her lover from two property developers. But she demanded McCambley give her a £5,000 cut. Her greed was astounding. The Robinsons reportedly earn almost £600,000 a year from politics. When the affair ended, Iris seems to have been petty and vindictive. She ordered McCambley to repay the £45,000. She set a deadline.

The image of Iris as Peter's meek, subdued wife is nonsense. Here was a woman mercilessly wielding her authority over a powerless young man.

And this wasn't Iris Robinson's first affair, a one-off mistake as a result of mental instability. The Sunday Tribune has been told she previously had an affair with McCambley's father, Billy, who had a drink problem. There was another affair in the 1980s with a DUP member. After Peter Robinson led a loyalist invasion into Clontibret, Co Monaghan, the RUC withdrew security from his east Belfast home. Off-duty security force members provided the Robinsons with protection. The DUP member, whose name is known to this newspaper, regularly visited Iris when Peter wasn't at home. They had numerous sexual encounters. We are not publishing his name to protect his privacy.

Sources have also said Iris once visited the Reverend Ian Paisley and said she wanted to leave Peter. Paisley advised her to try to make the marriage work for the sake of their children.

It was never the case that Iris didn't love her husband. She was 16 years old, taking a shorthand and typing course at Cregagh Tech, when she set eyes on Peter. From that moment, she knew he was "the one". He was a year older, studying English and Maths.

"He was very handsome. He stood out from the other boys," she told me. "All the girls danced attention on him. My strategy was to ignore him. I'd walk past him, nose in the air. It worked."

The marriage brought three children – Jonathan, Gareth and Rebekah – all well-mannered, well-adjusted individuals of whom any parent would be proud. They must be deeply hurt by what has transpired.

Iris suffered severe post-natal depression after Jonathan was born, of which she has courageously spoken. She loves her children deeply. When Rebekah's marriage broke up, Iris was her daughter's rock.

Despite her visit to Paisley, Iris never publicly admitted any domestic discontent. She always presented hers as the perfect marriage. Everything about Peter was "brilliant". Any time we chatted, she'd tell of some expensive gift he'd just bought her.

But the impression lingered of a lonely woman whose husband led a separate life. She'd complain that, while she realised he had a duty to "the province and the party", he was always away from home, and when he was home, he was locked in the study or on the computer.

Publicly, the impression was given that the marriage remained very much alive. In April 2008, as they posed for photos for the Sunday Tribune in the grounds of Stormont, the wind blew Iris's red dress up her legs. "It's your Marilyn Monroe moment!" Peter quipped.

Yet Iris's attention would soon be elsewhere. The café that Iris enabled her lover Kirk McCambley to buy that summer is near Shaw's Bridge on the River Lagan. The place should have held fond memories for her. It was by the banks of the Lagan, in his father's car, that Peter proposed.

But despite her affairs, there is no doubt Iris still adored her husband. She was politically loyal to the point of paranoia. When Paisley announced he was stepping down as DUP leader, Robinson, as his deputy, was his natural successor. No one else in the party expressed an interest in the leadership but Iris was not convinced that a lastminute rival wouldn't emerge.

"You don't know what Nigel [Dodds] is like," she told me. "He's always been jealous of Peter and he's so competitive."

Iris is a generous host. When my daughter Alanna was born, she invited us for lunch. She cradled the three-week old baby as though she was her own. She bought her a hand-painted tea-set. "From Auntie Iris," the card inside read.

Iris is a woman's woman. She'd curl her legs up on a chair, mug of coffee in hand, and spill her heart. She recalled one row with Peter when he was teaching her to drive: "He raised his voice. I pulled over, got out of the car and told him I'd make my own way home. I didn't think he'd let me but he just drove off and I'd to walk miles."

She was brutally honest in discussing her depression following her health problems and hysterectomy at 34. Yet she could be neurotic and highly strung. Once, she'd spotted me in Forestside shopping centre and thought I'd blanked her. I hadn't even seen her. Peter raised the matter with me, saying she was deeply upset.

Another time, she burst out crying in the middle of Marks and Spencers, saying Paisley, then First Minister-in-waiting, had never liked her or Peter and that he should be making her a minister in the new Stormont government.

She had odd ideas too. She told me the Robinsons could rarely eat out: "We have to be very careful. About 10 years ago, we had a meal in a restaurant on the outskirts of Belfast which had a very nationalist staff. Peter ordered steak. They put rat poison in it. He suffered bleeding and was extremely sick."

She was obsessed with clothes. She noted that Ulster Unionist MP Sylvia Hermon, who once was "so dull wearing grey and black is now copying my style". She complained when male DUP MPs were too friendly with Hermon on the Commons' benches.

Peter Robinson has been entirely faithful to Iris during their marriage. But Iris could be jealous for no reason. DUP colleagues teased her about one attractive female civil servant who worked with Robinson at Stormont. Iris climbed the walls with envy.

Yet within the DUP, she strongly encouraged other women. While she said Peter favoured Simon Hamilton to succeed her as Strangford MP, Iris supported Assembly member Michelle McIlveen. "I so much want to hand my seat on to another woman," she said.

Iris's constituency work in Strangford was first-class. "I fight for my constituents like my life depended on it," she said. On the campaign trail, she was magnificent. While Peter didn't do small talk, Iris was truly a people person.

The Robinsons' home – a huge white villa in east Belfast – is Iris's pride and joy. The opulence is overpowering: curtains of wine and gold silk rising into a central coronet, towering Chinese vases, hundreds of priceless antiques, and chandeliers in every room. "I think I was born in another era," Iris said as she gave me a tour.

Each room is themed. The dining room is oriental, the sitting room old English, the bathroom Italian, and one bedroom was French. Iris designed them all herself. She hired an artist to paint frescoes – a Tuscan landscape in the bathroom, an African one in the porch.

A massive four-poster Gothic bed with heart-shaped cushions dominated the Robinsons' bedroom. Iris had her own dressing room. Black lacy underwear was laid out for a function she was attending later. She opened her "bra drawers" to reveal row upon row of sexy lingerie. "Peter has over 1,000 ties and I have as many bras," she said.

Iris said she had no emotional feelings for her teenage lover. Knowing her, it's impossible to believe it was a purely physical and not romantic relationship. But given her off-handedness, Kirk McCambley may now feel he no longer needs to keep the most intimate details of their relationship secret. Perhaps Max Clifford will step in in a bidding war for his story.

Any such disclosures would be politically irrelevant. DUP moralists might disagree, but the wrongdoing in this story revolves around money, not sex. It's about how that money was obtained from builders – Iris was lobbying on behalf of a scheme for one of the builders – and how she failed to declare her financial interest in Kirk McCambley's business to the authorities.

There's also her breathtaking hypocrisy. She denounces homosexuality as "an abomination, sick, disgusting, nauseous and vile", as she has an affair with a teenager.

Three months ago Iris herself said: "If things are going on which are morally wrong... people have got to stand up and become whistle-blowers." Yet she covered up her own wrongdoing and dragged her political adviser, Selwyn Black, into her personal mess.

The Robinsons' moral pontification, and the flaunting of their wealth, has irritated many people over the years. On the surface, they had it all – the flash home, the holidays in Dubai and Florida, the dream life. But compare them to Ian and Eileen Paisley. One is a marriage of style; the other of substance.

After all these decades, the mutual affection and passion between the Paisleys is evident. He calls her "the Boss" and is never away from home without phoning to say goodnight. In the DUP, the Robinsons cornered the glitz and glamour. The Paisleys had the real love affair.