MR JOHN Gilligan was being held at Heathrow Airport in London last night after being arrested by British Customs officers under British drug trafficking legislation.
Mr Gilligan, who has said gardai suspect him of involvement in the murder of the journalist Veronica Guerin, was carrying £300,000 in cash and was boarding a flight to the Netherlands with another man when he was arrested.
A British Customs spokesman said its officers would "accede to any reasonable request" by British or Irish police to have access to the two men, when their investigation into the source of the cash is complete. Gardai are expected in London today to assist in the inquiry.
The man detained with Mr Gilligan is Mr Michael Cunningham, who was sentenced to 14 years imprisonment in 1986 for the kidnapping of Mrs Jennifer Guinness. He was released from Portlaoise Prison earlier this year. His brother, John, convicted of the same crime, absconded from prison last month and is still at large.
The money was found in Mr Gilligan's hand luggage at about 2 p.m. British Customs said it was knot illegal to export large amounts of cash, but the two were detained as they did not offer a satisfactory explanation for having the money.
Mr Gilligan can be held for up to 36 hours before the case has to be brought before a magistrate.
Gardai in Dublin said there are no outstanding warrants for Mr Gilligan in the Republic. However, the Garda will be able to supply information to the British Customs officers investigating the source of the £300,000, and any possible links to the drugs trade.
A major Garda investigation begun after Ms Guerin was shot dead in June has focused on a criminal empire headed by two men and involving drugs and money laundering. More than £100,000 has been seized by gardai during the investigation. Nine people were still being detained in Dublin last night after their arrest on Saturday under Section 30 of the Offences Against the State Act.
Mr Gilligan, who has convictions for robbery and receiving stolen goods, has spent most of his time abroad since Ms Guerin's murder.