Ratings agency Standard and Poor’s has held its credit rating for Ireland at BBB+, with a negative outlook.

In a ratings note, S&P says the Irish government has responded in a "proactive and substantive way" to the deterioration in the public finances and the crisis in the banking sector.

It also noted the achievement of a greater reduction in the budget deficit last year than called for in the EU/IMF programme.

It says the dynamics of the government debt burden have followed the path the agency expected, but the rebalancing of private sector balance sheets will continue to act as a drag on domestic growth.

It says the ratings outlook remains negative (as it does for most Eurozone countries) because of the still substantial task of reducing the deficit to 3% by 2015 - a task that could be undermined by weaker than expected growth in Ireland's main trading partners.

It says there is a one in three chance that Ireland’s ratings could be downgraded during 2012 or 2013.

In particular it says that the referendum on the fiscal treaty could lead to a ratings downgrade in the short term.

S&P's head of European Ratings, Moritz Kraemer, said the referendum on the fiscal treaty set for 31 May was "one of the flashpoints to observe in the coming months."

The ratings agency note says "It is our understanding that if the Irish electorate does not pass the amendment, Ireland would likely be excluded from the ESM's financial assistance programmes.

“As a result we believe that the outcome of this vote could affect Ireland's creditworthiness.”

"We believe that if the government cannot access the ESM [Europe’s permanent crisis resolution mechanism], this could exacerbate Ireland's funding difficulties when its current programme expires at the end of 2013.

“As a result Ireland could need additional official financial support. We consider it unlikely that the IMF or other potential providers of official funding would agree to fully finance a successor programme without significant co financing from Ireland's eurozone partners.

“After 1 July 2012 this would most likely have to happen through the ESM".

But S&P says there is still a chance that European partners could continue official funding even if the electorate were to vote No to the fiscal treaty, although this would be as a result of political negotiations.

For this reason it says that any downgrade as a result of a No vote would keep Ireland with an investment grade rating.

However, it adds "If we were to conclude that Ireland would be effectively excluded from future official funding before regaining reliable access to market funding, we could lower that rating to speculative grade.”