Irish bond yields dipped to a record low today after Standard & Poor's upgraded the country's credit ratings.

This added to the buoyant mood in peripheral euro zone debt triggered by fresh European Central Bank stimulus last week.

Other peripheral euro zone bond yields also hit the latest in a series of historic lows, with Spanish 10-year yields falling below those of US Treasuries for the first time since April 2010. 

Italian five-year yields were also below US equivalents, highlighting the policy divergence between the ECB and the Federal Reserve, which is reining in its monetary stimulus. 

ECB executive board member Benoit Coeure said at the weekend that euro zone interest rates would diverge for a number of years from those in the US and Britain, whose central banks would at some point raise rates. 

S&P's upgrade of Ireland's credit standing to A- from BBB+ on Friday evening fostered the euro zone rally.

The ratings agency said it could upgrade Ireland further if data confirmed the country's economic recovery and that fiscal deficits had fallen below 3% of gross domestic product. 

S&P also affirmed Italy's rating at "BBB/A-2" with a negative outlook. Moody's is scheduled to review Italy on Friday. 

Irish 10-year yields fell six basis points to 2.39% with Spanish yields down by a similar amount to an all-time low 2.59%, while Italian equivalents were 4 basis points lower at 2.71%. 

Last week's easing measures by the ECB have given fresh impetus to a two-year euro zone debt rally that has driven borrowing costs in countries that were at the forefront of the sovereign debt crisis to record lows. 

The ECB cut all its main rates, and ECB President Mario Draghi also outlined a new long-term loan programme (TLTRO) for banks to promote lending to small and mid-sized businesses.  

"The market now expects a reversal of the shrinkage of the ECB balance sheet, rates to stay lower for longer and there's also the possibility of further action and that's fuelling the rally," a trader said. 

At the euro zone's core, German Bunds further outperformed US and UK counterparts, driving the 10-year yield gaps to 2005 and 2010 levels respectively. 

Some analysts said banks were likely to use the four-year TLTRO loans to buy shorter-dated peripheral euro zone bonds and repay the money two years later, as there was nothing yet in the programme's conditions to dissuade them from such trades.