* Investors buoyed by S&P ratings lift
* PM Samaras says Greece will not need third bailout
* Fed meeting, Scotland vote pose volatility risks
* Spain bonds claws back ground after torrid week
* Supply outlook weighs on Ireland after IMF repayment plan (Adds fresh quote, updates prices)
By John Geddie
LONDON, Sept 15 (Reuters) - Greek bond yields edged lower on Monday after a credit rating upgrade from Standard & Poor's, which said the country remained on track to emerge from a six-year recession.
The upgrade to B from B- late on Friday is a boost for Greece's fragile coalition government, which is hoping to escape the constraints of its EU/IMF bailout programme.
Greece is expected to hold negotiations with its lenders on further debt relief later this year, and Prime Minister Antonis Samaras told a weekend newspaper he is confident the country will not need a third bailout.
Greek 10-year bond yields dipped 3 basis points to 5.70 percent at Monday's open, before paring some of those gains during the morning session.
"The upgrade was by-and-large expected, but it explains the slight outperformance this morning," said Rainer Guntermann, a rates strategist at Commerzbank.
Fitch raised Greece's rating to B in March and Moody's upgraded it to Caa1 in August. In keeping with the broad trend of ratings upgrades for peripheral Europe, analysts are now predicting Moody's will lift Slovenia's rating when it reviews the country on Friday, pulling it up into investment grade.
Before then, markets are awaiting the U.S. Federal Reserve's meeting on Wednesday for hints of when it may raise interest rates in response to the country's economic rebound.
Any rate hike - which would be the first in more than eight years - would ripple across global markets, sending borrowing costs for euro zone countries higher despite the ultra-loose monetary policy employed by the European Central Bank.
More immediate market volatility in the euro zone could stem from a closely fought referendum on Scottish independence from the United Kingdom being held on Thursday.
Analysts say a 'Yes' vote to break away could hurt bond markets in countries like Spain and Belgium which also have separatist movements, resulting also in an investor flight towards safe haven assets such as German Bunds.
One weekend poll showed the No vote 8 points in front, while another showed the same lead for the Yes camp and two others a 51-49 percent and 53-47 percent split respectively in favour of sticking with the union.
Spanish bonds clawed back some ground on Monday, after suffering their worst spell in more than a year last week. 10-year yields were 1 bps lower at 2.35 percent.