O'Brien struck France lock Pascal Pape in the first minute of the Pool D game on Sunday that sent Ireland into the last eight as group winners.
It was another huge blow to Ireland's chances of reaching the semi-finals for the first time after flanker Peter O'Mahony and captain Paul O'Connell were ruled out of the tournament through injury.
Number eight Jamie Heaslip will take over the captaincy from O'Connell and Chris Henry can play at flanker but the Irish may be forced to call in Rhys Ruddock who was added to the squad as a replacement for O'Mahony.
O'Brien admitted committing an act of foul play and has 48 hours in which to appeal.
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(Reporting by Ed Osmond; editing by Ken Ferris)
