Brian and Mary Pat O'Donnell say Bank of Ireland is trying to make them sell a prime London asset without legal entitlement

A lawyer and his doctor wife who blazed a trail through Ireland's property boom on the back of colossal credit – buying up several London trophy buildings on the way – borrowed together with a number of companies in which they have shareholdings more than €800m (£670m) from banks around the world, it has emerged.

The eye-popping scale of Brian and Mary Pat O'Donnell's debts has been laid bare in a court case brought by Bank of Ireland in relation to some €70m (£58m) in debts it is trying to recover.

The couple, from Killiney, county Dublin, are fighting the case and say the bank is trying to make them sell one of their prime London assets – Sanctuary Buildings, used by the Department for Education – without legal entitlement.

The office block is just metres from the Houses of Parliament and was the first of many audacious moves made by the solicitor and his wife into the property market at a time when Ireland's banks were making huge loans that have since brought the country to its knees and forced Dublin to accept a bailout from the EU and IMF.

The Bank of Ireland loaned the O'Donnells €26.7m towards the £170m purchase price in 2006. The couple then went on to expand their empire to Stockholm, where they bought the city's biggest office block, Fatburen for €285m.

In April 2008, when the worldwide credit crunch was beginning to bite and had already claimed the likes of Bear Stearns in New York, the couple were still able to access funds.

With the strength of the euro in their favour, the O'Donnells managed to outbid a group from Dubai to pay a record $172.5m (£87m) for an office building on Pennsylvania Avenue, Washington DC, just a few blocks from the White House.

This was what the O'Donnell's Vico Capital described as "trophy class" with "sweeping views of the White House, the Monuments and the Potomac River". Its tenants included a law firm and a merchant bank and it was described in the local property press as a smart buy.

Vico Capital set a record, paying $867 (£436) for every square foot of the their Washington investment. The previous record in the American capital was $827.

Arguably, even more audacious was the acquisition of two buildings in London's Canary Wharf – the purchase of 17 Columbus Courtyard in 2005 for £125m and 15 Westferry Circus for £140m. The first building is home to Credit Suisse and the second to Morgan Stanley.

For years, it probably seemed nothing could go wrong. The O'Donnells live in a clifftop home in one of Ireland's most salubrious suburban roads and count the U2 frontman Bono among their neighbours.

The O'Donnells' rise in the property sector mirrors that of dozens of other investors who got sucked in during the Celtic Tiger years when credit was cheap and capital yields, or profit on buildings, meant millions could be made, often in just a number of weeks.

But unlike many other middle-class professionals of their ilk who dabbled in the property market with a few buy-to-lets, this couple became major players and amassed an international property portfolio of more than €1.1bn (£921m) with a rent roll of some €150m (£125m).

They are, however, a low profile couple, and are aghast that their private financial affairs are now being made public. It is understood that as recently as last weekend, they tried to get a court approved mediation process underway.

"Most people in Dublin wouldn't know what they looked like. They are an extremely private family and would have hoped to have continued to conduct their business in private," said a source close to the couple.

Their case isthe latest in a series of court actions being instigated against property developers now bearing the brunt of Ireland's financial collapse.

According to informed sources there are two other private investors in the O'Donnells' Sanctuary Buildings in Westminster and there are concerns that the court case will lead to "value destruction". However the O'Donnells are determined to tough it out and have been given three weeks to put together a case for a fuller court hearing.

O'Donnell, 58, is one of Dublin's leading commercial lawyers and ironically is on a list of 64 potential legal advisers approved by Ireland's National Asset Management Agency – the new state bank which has been charged with clearing the mountain of bad debt amassed by property developers during the good times. Nama said O'Donnell had not been used in any case.

His practice, Brian O'Donnell solicitors, has the usual run of commercial expertise including mergers and acquisitions, corporate restructuring, insolvency and tax structuring. His practice, however, is not high profile in the media.

He first diversified into a serious property business back in 1999 and, with access to funds from a string of banks including Bank of Ireland, Ulster Bank and Anglo-Irish, made his audacious moves in London, Scandinavia and the US.

Ten years later he and his wife, now 56, who are relatively low-key on the Dublin social circuit, were listed on the Sunday Times Rich List – 178th richest in Ireland, alongside another, more high profile property investor Derek Quinlan.

Quinlan, a former tax inspector who also caught the property bug, ended up with the crown jewels of London's hotel and retail trade including the Savoy Hotel, Claridges and a retail block between Harvey Nichols and Knightsbridge in London.

• This article was amended on Sunday 23 January to correct an outdated reference to Department for Education and Skills.

• This article was amended on Tuesday 6 December 2011. The original headline and opening paragraph wrongly stated that the O'Donnell's owed €800m. In fact, that figure was the total amount of their combined borrowings with a number of companies in which they had shareholdings. Further, since this article was first published, the legal dispute brought by the Bank of Ireland against the O'Donnell's has been settled on undisclosed terms.