They were described by one investor as "being like Siamese twins", but this week it emerged businessmen Greg and Hugh Kavanagh were after having a very serious falling out.
The bust-up between the two is said to have not only been physical, but also involved the alleged unlawful removal of Hugh (40) as a director of companies they are involved in.
What was a very close personal relationship had deteriorated spectacularly, the High Court heard.
In a legal filing, Hugh Kavanagh said the break-up of his relationship with his brother had caused him "immense personal grief and distress".
Of the two businessmen, Greg (35) is the better known.
The high-profile Arklow-born developer once described himself as the Ronaldo of the property market and the nickname stuck.
He co-founded New Generation Homes with Pat Crean in 2011 and went on to make a fortune.
With the backing of investors, the company is thought to have spent as much as €300m buying sites around Dublin at knock-down prices following the financial crash.
Greg exited the business in 2016 and is reported to have received as much as €150m as part of the deal surrounding his departure.
He has still remained in the property business though.
More recently he has also been backing a new fashion retail app called Bezzu, which allows small clothing and homeware outlets to sell their products online.
His marriage to financial adviser Sheila Martin catapulted him briefly from the business section to the social pages in 2018.
The couple had a lavish reception at Adare Manor in Co Limerick during which guests were treated to a performance from chart-topping English singer Ellie Goulding.
One wedding planner dubbed the event "Harry and Meghan 2.0".
Last year his wife's family-run business Betty Martin Financial Services hit the headlines after it secured a High Court injunction preventing AIB subsidiary EBS from terminating its tied agency agreement.
The court heard claims the agreement was terminated over the business's refusal to engage in the misselling of financial products to customers. EBS denied the claims.
It was reported at the time that Greg liaised with senior AIB executives in an attempt to resolve the matter before it ended up in court.
Although christened Bernard, his older brother goes by the name Hugh.
According to his barrister Rossa Fanning SC, while Greg was seen as the "front of house" of the business, Hugh had the "hands-on" role in terms of actual building work and development activity.
In a legal filing, Hugh said he and his younger brother had operated a successful property and construction business together for 17 years, working closely together. Hugh maintained they were always 50/50 business partners.
"We started out together in 2004 and from then on had shared everything," he said.
"There was never a need to formally document our agreement. It was simply entirely natural. He always had my back and I always had his."
Hugh Kavanagh said one investor described them "as being like Siamese twins".
Important decisions were made jointly and it was always agreed they owned the business equally.
But all of that changed during March and April this year.
Company Registration Office filings record Hugh as having "resigned" as a director of 19 of their companies on April 29.
But he claims he did not resign and was unlawfully removed by his brother.
Tensions between them over business issues appear to have boiled over the previous month.
According to Hugh, a night out in Dublin city centre ended with a physical fight between them.
The construction side of the business was experiencing cash-flow issues as funds were being channelled into the fashion app business.
Hugh had been dealing with building contractors who were anxious to be paid.
Details of the dispute emerged on Tuesday, when he went to the High Court.
Represented by Mr Fanning, one of the country's leading commercial law barristers, and solicitors from 'big five' law firm McCann Fitzgerald, he was granted leave to serve short notice of the proceedings on his brother and 20 companies. The court heard he had been removed as a director of all but one of these companies, Bezzu Corporation Ltd.
These firms included the main holding company in the business, Structured Marshalled Investments Ltd (SMIL), which Hugh estimated had a net value of €35m.
In an affidavit, he said he wished to emphasise he was not bringing the proceedings lightly, but believed he had no other option.
Ultimately, his action seeks declarations from the court confirming he has an ownership interest in SMIL and that he remains a lawfully appointed director of the companies he was removed from.
He also wants injunctions, pending trial, preventing his removal as a director of the companies, permitting him to continue to participate in managing the business, and ensuring the value of his stake in the business is not diluted.
In his legal filing, Hugh said the brothers had a very close personal relationship until about six weeks ago.
To illustrate this, he spoke of how they were each best man at the other's wedding.
He said Greg was also godfather to two of his three children.
According to Hugh, their business relationship was formalised in 2018 when 50pc of the shares in SMIL were transferred to his holding company, Simlur Limited.
But in recent months the relationship deteriorated dramatically.
According to Hugh, his brother was now asserting he had no ownership interest in SMIL or other group companies.
He claimed his brother had purported to remove him as a director of the 19 companies in a manner which was "clearly unlawful".
Ms Justice Deirdre Murphy agreed the matter was an urgent one and adjourned it to yesterday.
The court has yet to hear Greg Kavanagh's side of the story and it is not certain that it will. Both sides appear keen to resolve things before there is a full hearing.
By Thursday, discussions were taking place between advisers for the two brothers and that night there was a suggestion the dispute had been resolved. This would prove to be premature and the court was informed yesterday that talks were ongoing.
When no resolution was agreed by 4pm, the matter was adjourned again until Monday to allow further negotiations over the weekend.
Things are understood to remain tense between both sides.
Hugh Kavanagh did not respond when contacted by the Irish Independent for comment, while Greg could not be reached.
Greg has weathered legal storms before, but none as close to home as the dispute with his brother.
In a 2018 judgment in a case over the ownership of small parcels of land in the Dublin docklands, a judge described him as a "generally unreliable witness" who "deliberately misled the court on an issue going to credibility".
The previous year Greg Kavanagh became embroiled in a multi-million euro dispute with Gerry Mulvey, a director of League of Ireland football club Bray Wanderers.
Both sides claimed the other breached the terms of an agreement under which construction and property company shares were to be transferred.
The matter was settled out of court.