The IRA used the Irish Republic's Celtic Tiger boom to buy up properties in London, Dublin and Spain, according to leaked US cables from Wikileaks.
A senior Irish police officer told the US embassy in Dublin that the IRA used the booming economy to diversify.
They moved on from 1970s-style racketeering, turning into "apparently respectable businessmen" to raise cash.
The cable was by Jonathan Benton who was then deputy chief of mission at the US embassy in Dublin.
He said a senior Irish policeman commented that IRA money was constantly moving diverse sources into a wide range of investments.
The cable said they IRA had moved from "smuggling, robbery and racketeering" into "more sophisticated enterprises." These included building up a portfolio in real estate.
The cable said the money was being used to support Sinn Fein's political activities in the Republic of Ireland.
"Irish officials, more generally, remain concerned that IRA funds acquired through sophisticated investments are seeping into resources available for Sinn Fein's political activities in the Republic of Ireland."
But Sinn Fein said this was "complete nonsense".
A spokesperson pointed out: "Sinn Fein is the only party (in the Irish republic) to publish its accounts before legislation was passed requiring others to do that."
Given the current Irish economic crisis, a cable dating from 2004 refers to the Irish economic boom.
James Kenny, the US ambassador to Ireland at the time, reported that Washington was so impressed by the growth of the Irish economy that the US treasurer from that time, John Snow, visited Ireland.