IOC spokesman Mark Adams says the athlete is alleged to have "bet against themselves prior to the Olympics." He says the IOC is working with the Irish Olympic Committee to gather information.
IOC rules bar athletes from betting on Olympic events, though Adams says the case appears to have "no effect so far on the games."
The Irish Independent reports that an athlete made a $4,800 profit and withdrew $4,435 cash from an account with a gambling operator.
The athlete and sport are not identified.
The IOC has registered 66 athletes from Ireland at the London Olympics.