LONDON, May 16 (Reuters) - Ireland's government borrowing costs fell to their lowest level in over a month on Monday after Moody's upgraded the country's credit rating.

Moody's, the only agency to cut Irish debt to junk in 2011, raised its credit rating to A3 early on Saturday, meaning Ireland has won back an 'A' category rating from all the major agencies.

When markets re-opened on Monday, Ireland's 10-year yields fell below 0.80 percent for the first time since April 11, down 4 basis points on the day.

(Reporting by John Geddie, editing by Nigel Stephenson)