Credit rating agency Fitch has put Ireland's top AAA rating on 'rating watch negative', which means it is considering lowering the rating. A lower credit rating usually makes borrowing more expensive.
Earlier, however, it emerged that US bank JP Morgan had advised its clients not to bet that Ireland would be unable to pay its debt, describing the country's financial position as 'remarkably strong'.
Fitch said it had taken its decision after the recent Exchequer figures, which showed sharp falls in tax receipts in the first two months of the year.
It said this would intensify the policy challenges facing the Government as it seeks to bring the public finances under control.
The agency said its examination of the prospects for the public finances would include the worsening tax figures and the Government's supplementary Budget measures. It said that in such cases it usually made a decision within three to six months.
Two other agencies - Standard & Poor's and Moody's - have already put Ireland's AAA rating on negative watch.
Credit rating agencies rate the ability of companies and countries to meet their financial obligations and their reports are watched by those who lend money in capital markets.
Meanwhile, in a research note, JP Morgan said many other European countries had significantly worse public debt to GDP ratios than Ireland. JP Morgan also said the Government had 'significant capacity' to service a large increase in its debt.
On the costs of supporting the banks, JP Morgan said that though this would require more funding, it was likely to be 'costly rather than impossible'.
The bank also said it saw value in the bonds of the six banks covered by the Government's guarantee scheme up to September 2010, while the bonds of AIB and Bank of Ireland also offered 'compelling value' in the period after the guarantee.