Irish police have recovered a major portion of the €7.6m taken in a tiger kidnap yesterday - the biggest heist in the Republic's history.

Last night six men and a woman were in garda custody after detectives stopped at least two cars and searched a number of houses in Dublin.

Although the cash had not been fully counted, initial estimates suggested that up to half of the stolen haul had been recovered.

The breakthrough came following a major security operation, spearheaded by members of the force's organised crime unit and the national surveillance unit.

The operation was aimed at an inner-city gang, which had been one the prime suspects for the kidnap and robbery.

One of those detained by gardai is believed to be a relative of a former gang boss, who had been the target of the Criminal Assets Bureau as a result of a series of major armed robberies several years ago.

The relative had been questioned by gardai earlier this week in connection with a murder and then released without charge.

The bulk of the recovered money was found in a car, which was intercepted by armed detectives following a chase through Blanchardstown.

The cash was in a holdall bag in the bag of the car.

Gardai detained two men in the car and brought them for questioning to Blanchardstown garda station under section 30 of the Offences Against the State Act.

Officers said the two men were well known to gardai.

Another car was stopped in Phibsborough and a substantial five-figure sum found in the vehicle.

Two other men were arrested there and a house searched in the area.

The big operation, organised by assistant commissioners Dermot Jennings and Al McHugh, was launched within hours of the heist from the Bank of Ireland branch at College Green in the centre of Dublin.

Gardai confirmed last night that a further two men and a woman were detained elsewhere in the capital, bringing the total number of people arrested so far to seven.

A number of vehicles were seized for technical examination and several houses searched during the operation.

Gardai sealed off a small red bricked terraced house at Great Western Villas, off Monk Place, Phibsborough, Dublin, last night.

They had earlier taken away two cars for examination, a 2006 BMW 520 estate, and a 1999 Volkswagen Golf which were taken on a tow truck to Santry garda station.

But a major inquiry was under way last night into the collapse of internal bank security, which allowed the gang to carry out the heist.

Members of a city crime gang, specialising in bank robberies, were believed to have been responsible for the heist.

The failure of bank officials to alert the gardai in accordance with agreed security measures was criticised by Justice Minister Dermot Ahern after separate talks with the chief executive of the Bank of Ireland, Richard Boucher, and senior garda officers.

The minister said he was shocked at the amount of money taken and the breakdown in procedures.

Early-morning staff at the Bank of Ireland branch at College Green, in central Dublin, agreed to help their distraught 24-year-old colleague Shane Travers fill four plastic bags with cash after he had shown them a photograph of three hostages being held at gunpoint.

But questions were being asked last night about why they had stuffed so much money into the bags rather than fill them with smaller denominations.

Mr Ahern has also asked why the gardai were not alerted about the incident until Mr Travers had surrendered his car with the cash and then walked into Clontarf Garda Station.

Mr Travers, who is from Portmarnock, Co Dublin, has been working with the bank for the past two years. His father is a garda, based at Clontarf station. The incident began around 10pm on Thursday. Mr Travers was watching TV at the home of his girlfriend, Stephanie Smith, at Stonebridge House, Badger's Hill, between Kilteel, Naas, Co Kildare and Rathcoole, Co Dublin. Ms Smith had gone shopping with her mother, Mrs Joan Smith and then collected her six-year-old nephew, whose parents were flying to Spain on holiday.

Mr Travers heard the sound of a car pulling into the driveway and assumed it was the return of his girlfriend.

As the women and child entered the house, the gang jumped out of the bushes and grabbed them.

During a struggle one of the thugs struck Ms Smith viciously on the back of the head with a vase. The six raiders were all masked in balaclava helmets and armed with two handguns and a sawn-off shotgun. The raiders confiscated their hostages' mobile phones and held them in the house overnight.

At around 5.30am three of the gang bundled Ms Smith, her mother and nephew into a blue Peugeot Partner van and drove them to a derelict house at The Rath, near Ashbourne, Co Meath. They also drove Mrs Smith's blue Volkswagen Gold in the direction of Dublin.

Meanwhile, the rest of the gang remained behind with Mr Travers and gave him a mobile phone to receive instructions. He was handed four plastic bags and told to take them to his workplace at College Green and fill them with €20, €50, €100 and €200 bank notes. He was also supplied with the photo of the hostages at gunpoint to convince his colleagues that their lives were in danger.