Despite being sidelined by injury for the tense 27-26 defeat to La Rochelle, the Leinster five-eighth made his way onto the field following the match and was captured on video engaging in a heated discussion with the officials.
"Johnny Sexton admitted misconduct. The Disciplinary Committee found his behaviour confrontational and aggressive towards, and disrespectful of, the match officials, organisers," European Professional Club Rugby said on Sunday.
Leinster were fined Stg 7500 ($A14,300).
Ireland, the world's top-ranked team, have three warm-up matches next month: against Italy on August 5, England on August 19 and Samoa on August 26.
Sexton is, therefore, free to play from August 27.
Andy Farrell's men open their World Cup campaign in France with their two easier pool games against Romania and Tonga, before taking on holders South Africa on September 23 and Scotland on October 7.
The quarter-finals begin a week later on October 14.
Sexton is working his way back to fitness after requiring surgery on a groin injury sustained during Ireland's Six Nations grand slam-securing win over England in March.
Ireland are hugely reliant on Sexton, the linchpin of the national team for more than a decade who turned 38 this month.
While his understudy at Leinster, Ross Byrne, impressed when called upon during the Six Nations, and Jack Crowley had a strong finish to Munster's United Rugby Championship winning season, neither are at the same level as Sexton.
While the ban will impact the Irish skipper's preparations, he has rarely needed much time to get up to speed following previous injuries and barely featured for Leinster last season before leading Ireland to the grand slam.
