The money has been handed over as part of a long-standing arrangement to fund treatment for around 50,000 pensioners who worked in the UK.
But the Conservative Party has said the process is so 'shambolic' that up to €200m (£180m) a year too much has been paid out.
The issue emerged following comments by Minister for Health Mary Harney in the Dáil in December.
She said the UK Government 'reckoned they had paid us €150m more than they should have' in 2007, and wanted to claw back the cash.
Ms Harney predicted that the payment level - which is based on national insurance contributions made by Irish citizens - would go down to €250m annually in future.
Conservative Party health spokesman Andrew Lansley said he suspected the overpayments could be up to €750m over the past five years, from a total bill of €2bn.
He demanded a parliamentary inquiry, insisting ministers were trying to hide the problem. The British Department of Health has insisted the governments are in talks over the issue.
'Labour's handling of this situation has been yet another shambles,' Mr Lansley said.
'NHS funds are always precious and it's completely unacceptable for the Government to be carelessly diverting £180 million a year out of our NHS.
'There are so many different ways that money could be spent to provide better care for NHS patients.'
Mr Lansley said ministers in Westminster needed to take responsibility 'for the painful impact the confusion is having on the already overstretched Irish health system'.
