The Ireland international was cited by match citing commissioner John Byett for making alleged contact with the eye or eye area of Leinster captain Leo Cullen in contravention of Law 10.4 (k).
Law 10.4 (k) relates to acts contrary to good sportsmanship.
The hearing, which was also attended by Munster captain Paul O'Connell and team manager Shaun Payne, began at 1pm and lasted a number of hours.
The independent disciplinary committee, chaired by Roger Morris (Wales) and also comprising Simon Thomas (Wales) and Mike Hamlin (England), found Quinlan guilty of foul play in contravention of Law 10.4 (k).
After considering the evidence, the committee determined that the offence was at the low range of the level of seriousness for an offence of this type.
Having taken into account any mitigating and aggravating factors, the committee suspended the back rower for a period of 12 weeks starting today (May 13) and running up to and including, September 9, 2009.
The 12-week suspension is for 12 playing weeks with the off-season not included in the period of ineligibility.
As a result of his ban, Quinlan will miss the British & Irish Lions tour to South Africa and the first game of next season.
The Lions tour begins on Saturday, May 30 and the Tipperary man's suspension, following on from a season-ended injury to his Munster team-mate Tomas O'Leary, means that the Irish contingent in the Lions touring party is now down to 12 players.
Both parties (ERC and the player) have the right to appeal. Appeals must be lodged within 72 hours of receiving the full written decision from the chairman of the independent disciplinary committee.