Fifteen thousand taxpayers have less than two months to get their affairs in order after owning up to holding offshore accounts.

They will be the first focus of Revenue authorities who are tracking down hot money held in accounts outside the State or undeclared interest on clean money banked offshore.

The 15,000, who came forward before last Monday's deadline, will shortly receive letters of acknowledgment from the Revenue.

They will also be poring over spreadsheets provided by the Revenue on its website. These allow taxpayers or their accountants calculate their liabilities as far back as 1987.

Those who voluntarily disclose their liabilities face tax and levies ranging from 47-59 per cent depending on the year the money was earned. They also face statutory interest ranging from 18 per cent on undeclared income from 2002 to 230 per cent on money undeclared since 1987/88.

In addition, penalties will be imposed for non-payment. In the case of those who contacted Revenue before the deadline, this will be mitigated to one-tenth of their tax and levy liability in the relevant year since April 1991.

However, in earlier years, the Revenue has no authority to mitigate the penalties and so they will amount to a sum equal to the interest and levies due in those years.

The Revenue staff dealing with the investigation into offshore accounts reckon that dealing with those who have admitted their exposure to tax on such accounts will occupy their resources until approximately the end of June.

The Revenue will only then turn its attention to those who have failed to come forward. Tax officials acknowledge that tracking down offshore money might be more complicated than unearthing bogus non-resident accounts. However, they are confident that there should be fewer tax evaders to uncover.

"Before we began the inquiry into bogus non-resident accounts, using recently acquired powers, bank officials, accountants and their customers may have thought we could not get access to their accounts," said a Revenue spokesman. "They know different now."

Revenue officials will seek High Court orders under section 908 of the Taxes Consolidation Act 1997 in relation to the 10 financial institutions that recently got their foreign subsidiaries to issue 120,000 letters to account holders with Irish addresses. These relate to accounts held in Northern Ireland, Britain, the Isle of Man and the Channel Islands.

Revenue officials have yet to decide whether they will pursue accountholders at all 10 institutions simultaneously or whether they will take them one at a time. A spokesman said they would consult with the banks on the most efficient way to proceed.

Revenue officials will initially target those offshore accounts where money passed through the Irish clearing system at some stage. This would snare those who sent money abroad from Irish accounts or repatriated it at some stage.

Tracing these funds is easier because, like the inquiry into bogus non-resident accounts, it does not require the Revenue to seek access to information held outside the State.

Once this phase of the investigation is complete, Revenue will turn its attention to money spirited abroad and kept there.

Additional powers granted to the Revenue in the most recent Finance Act allow it to require Irish financial institutions to instruct their foreign subsidiaries to disclose any information they might have on accounts that they suspect might be held offshore.

This will come as a rude awakening to certain Irish clients of offshore banks who claim they have been assured that the banks would not supply information to the Revenue.

"The only thing I can suggest is that when a bank manager tells a customer something along those lines, they get the bank official to put it in writing," said the Revenue spokesman. "That should be the moment of truth."

Those caught by Revenue investigators will have to repay tax and interest in the same way as those who came forward voluntarily. However, they will not be able to mitigate the penalty element of the settlement.

In addition, they face the prospect of having their details published in Iris Ofigúil and in the national media.

They also run the risk of criminal prosecution, with penalties ranging up to fines of €126,970 or a prison term of up to five years on conviction.

"You are talking about cases where the amounts involved are significant and there has been no restitution," said the Revenue spokesman.

Those who cheated the taxman in previous tax amnesties will have those issues re-opened. However, the Revenue has repeated that people such as students, cross-Border workers and those with insignificant "shopping" accounts outside the jurisdiction have nothing to fear.

"The people we are after are those who knew what they were doing," a Revenue source said.