Number eight Koyamaibole was reported by citing commissioner John West for an act contrary to good sportsmanship following the incident during last Saturday's Amlin Challenge Cup pool game between Irish and Bordeaux-Begles in France.
An independent European Rugby Cup disciplinary committee, meeting in Dublin on Wednesday, imposed a suspension that means Koyamaibole cannot play again until January 7.
Independent judicial officer, England's Anthony Davies, heard evidence and submissions from Koyamaibole by conference call.
The player accepted that he had committed an act of foul play contrary to good sportsmanship.
Davies upheld the citing complaint, and ERC said in a statement: "The judicial officer determined that the offence, although intentional, had caused minimal injury, and was at the low-end entry point of 12 weeks.
"After considering any mitigating and aggravating factors he decided that the entry point was sufficient and imposed a suspension of 12 weeks.
"Koyamaibole will be free to play on January 7, 2013. Both the player and ERC have the right to appeal the decision."
Had the incident been deemed a top-end offence, then Koyamaibole could have been banned from anything between 24 weeks and four years, under International Rugby Board recommended sanctions.
Koyamaibole, who was represented at the hearing by Bordeaux-Begles rugby director Raphael Ibanez, will miss his club's next three Challenge Cup games, starting with tomorrow night's encounter against Gloucester at Kingsholm.
He will also sit out a number of French Top 14 league games and will not be available for Fiji's autumn Test matches, including a Twickenham appointment with England on November 10.
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