Last Monday, the earth shifted on its axis and Lenihan the Younger became a man. It was no optical illusion, when dawn broke on Tuesday and the finance minister flung open his office door to herald a new banking order, that he looked the spitting image of his father. His eyes told the story of the 49-year-old's belated coming of age. Hollow with exhaustion (and the loss of a lens from his reading glasses), they had the haunted look of a man who had seen the future, and was forced to change it for the salvation of humankind.

"He was three nights without sleep and he was snow white going into the Seanad on Thursday morning," says his aunt, Mary O'Rourke, who encountered him on his way into the Upper House with his bankers' guarantee bill under his arm. "The sight of him brought the mother out in me."

The corridors of power are the Lenihan clan's second home. Brian 'Don't call me part of the Establishment' Joseph is the grandson, the son, the nephew and the brother of four Fianna Fáil TDs. His father, the late Brian Lenihan, whose Dáil seat he inherited in a 1996 Dublin West by-election, had been tánaiste to Charlie Haughey. His aunt, Mary O'Rourke, was deputy leader to Bertie Ahern. Brian fils, the first-born of his generation of Lenihans, was being called to greatness from the cradle. Unlike banking apocalypse, his destiny was unavoidable.

But, boy, did it wobble. Since 7 May, when Brian Cowen made him his finance minister in the wake of Bertie Ahern's grudging promotion of him to the cabinet, Lenihan's future seemed firmly behind him, as the first-class honours Cambridge old-boy and senior counsel repeatedly demonstrated a facility for the art of the gaffe. He informed a conference at Dublin Castle, during an interval in Ahern's appearances at the Mahon tribunal, that the venue was where Ireland tried corrupt politicians. He then proceeded to bemoan the timing of his elevation to the worst job in the land. Even if he meant it as a joke, it wasn't funny. Moreover, while Brian Cowen is blessed in having Enda Kenny as his aspiring nemesis, Lenihan faces two of the Dáil's brightest members, Richard Bruton and Joan Burton, on the Opposition benches.

All summer long, rumours persisted that Lenihan – who, despite his academic braininess, barely scraped a pass in Leaving Cert maths – was drowning in the finance portfolio. The consensus was that Cowen, his predecessor in the department and his patron, was undermining his authority by dictating his every decision. The taoiseach and his finance minister became derisively known as "the two brains" as the pair of lauded geniuses presided as energetically as Lot's wife over the rapidly worsening economy.

What a difference an economic crisis – or a "third world war without bombs and bullets" (courtesy of an anonymous stockbroker) – can make to a politician's reputation. As last week wore on, Ireland's trading rooms were clanging with tributes to Brian Lenihan's liberated-at-last machismo. An aide, when asked how the boss was surviving on scant sleep, brushed it off: "He doesn't need much sleep. He's a night owl." The story is being told in informed circles of how he faced down Britain's chancellor of the exchequer, Alastair Darling, when the latter sought, with impassioned language, in his second phone call to Merrion Street that day, to dissuade his Irish counterpart from his guarantor strategy. In these pinch-me days of 1980s retrospection, any flexing of patriotic muscle can only gratify a resurgent creeping anti-Brit sentiment. According to those in the know, the more Darling exhorted "You can't do this, it's uncompetitive," the more composed the Irish minister sounded.

He has had plenty of practice suffering the excesses of politicians. As a volunteer on the presidential campaign in 1990, he watched his father being sacked by his "good friend" Haughey over the calls-to-the-Áras controversy, only to find out years later through the tribunals that Haughey had used his father's liver transplant fund to finance his own Medici lifestyle. Bertie Ahern had been his father's campaign manager in 1990. When he became taoiseach, he left the son idling on the back benches until he could no longer justify it. The tension between the two men continues. Lenihan was the only one of the cabinet at the height of Bertiegate to enunciate clearly that he would not have taken the so-called "whip-round" proceeds from Manchester. Ahern was bristling in the privacy of his Drumcondra bailiwick.

"Look, the most significant thing to shape Brian Lenihan's life was the sickness and death of his father," says a friend, speaking on condition that he will not be identified. "They were very close. When his father was in the US for treatment, Brian was the surrogate candidate in Dublin West. He manned his clinics for him when he was absent due to illness as a TD. And he took his lead from his father not to be bitter. I think he's very conflicted on Haughey. He recognises that, if the money hadn't been forthcoming for the operation, his father would not have got the treatment and he probably would not have had those extra years of life. But he may also be angry that the money was used for other purposes. And, yes, it's true that there has been some tension between himself and Cowen. Previous ministers for finance find it hard to let go. It causes a certain amount of tension and a certain amount of confusion among civil servants as to who is their boss."

The accusation that the state guarantee for the banks is a sticking plaster applied by Fianna Fáil, the Builders' Party, for the ultimate benefit of the construction industry, is one that cannot attach to Lenihan, argues his friend. "He'd never have been a Galway tent man."

Yet that is the legacy he must deal with, a fact starkly evident last Monday. The need for him to rescue the banks was motivated in large part by the indebtedness of the property sector. Among the biggest borrowers are the former Fianna Fáil county councillor, Bernard McNamara, and Seán Dunne, Bertie Ahern's VIP guest for his Westminster and Capitol Hill speeches.

Last week's events underlined the intimacy of the Irish establishment. Former Fianna Fáil general secretary Pat Farrell is the head of the Irish Bankers' Federation. One of Brian Cowen's foremost confidants is Paddy Power chairman Fintan Drury, who was a director of Anglo Irish Bank until last summer. The attorney general, Paul Gallagher, who attended the meetings on Monday night at Merrion Street, succeeded his good friend Dermot Gleeson as the highest-earning senior counsel in the country when the latter was made attorney general in 1994. Gleeson, now chairman of AIB, was also at Monday night's meeting where the legislative remedy for the banks' dilemma was approved.

"The famous Lenihan bonhomie goes on," says Labour deputy leader Joan Burton. "He's eerily like his father – 'what's the problem? No problem'. He's a person of integrity but can he wean himself off Bertie's relationship with the builders?"

On family holidays in France (they travel by ferry) with his wife, circuit court judge Patricia Ryan, and their two children, Brian Lenihan reads French books. It was his aunt, Mary O'Rourke, who taught him Latin when he was 12, the summer before his family moved from Athlone to Castleknock, Dublin, and Brian transferred from the Marists to the Jesuits in Belvedere College, where he was chosen as school captain.

"He was lovable but he was always a very serious child," says O'Rourke. When his friend is asked if Brian and his brother, integration minister Conor Lenihan, are close, he replies: "I'd say there are other people in the parliamentary party he might be closer to."

If last week was a test of the new finance minister's mettle in a crisis, his fast-forwarded debut budget next week is, according to a source, "what history will remember him for". The talking heads of the financial world, who have been massaging Lenihan's ego with their plaudits, are now urging him to cut taxes, spend more and generate market activity, despite Thursday's revelation that the economy has contracted by eight per cent in the past year. Aware of his own lack of monetary expertise, Lenihan is said to be heavily dependent on department of finance mandarins and has been canvassing his father's old cabinet colleague, Ray MacSharry, for advice. He is said to be not particularly close to Charlie McCreevy.

As the country holds its breath for the most important budget in two decades, it can only be hoped that the minister will suspend the famous Lenihan trait of concealing their intelligence.