Ronan suffered the initial setback in training last month but further investigation has led to an operation that will rule him out of the remainder of the Heineken Cup pool stages and a potential quarter-final.
"Niall Ronan, who underwent a knee scope last Friday, required cartilage repair work and has been ruled out for approximately four to six months," read a Munster statement.
"Niall sustained a knee sprain at training in November and met with the specialist on a number of occasions. Following a final review, it was confirmed he was not progressing as well as hoped and required the knee scope surgery."
Ronan featured in the opening Heineken Cup pool game against Edinburgh at Murrayfield but missed the following week's win over Gloucester and was injured prior to the back-to-back victories over Perpignan.
Losing a player of Ronan's experience is a definite blow for Munster, even if he has been used as more of a squad player rather than a regular starter. The 31-year-old has made more than a century of appearances for the province since moving to Thomond Park from Leinster at the start of the 2007/08 season.
Munster have been hit by a number of injuries in recent weeks and months, with the likes of Lions duo Conor Murray and Simon Zebo out of action.
Duncan Casey, Dave O'Callaghan and Tommy O'Donnell are following standard return to play protocols after heavy collisions in the late win over the Scarlets at the weekend; BJ Botha's knee problem is being monitored closely; Ian Keatley has only just returned to light training following a dead leg; and Damien Varley is recovering from illness.
Mike Sherry has recently had surgery on a knee injury, but wings Andrew Conway and Gerhard van den Heever both returned to action on Saturday night and Paddy Butler was back in training yesterday.