A statement on the Munster website (www.munsterrugby.ie) said the player had suffered medial ligament damage to his left knee during Sunday's Heineken Cup game against Edinburgh.
The 26-year-old underwent a scan on Monday before meeting a specialist in Dublin where the injury prognosis was confirmed.
Ireland begin their Six Nations campaign against Scotland on February 2 and finish with a game in France on March 15.
Earls has not played for Ireland since Joe Schmidt took over as coach in April, having missed the November internationals with knee problems.
(Reporting by Toby Davis; editing by Tony Jimenez)
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