The former Dundalk boss was primed to take the helm after UEFA Euro 2020, but the postponement of that tournament to next year amid the coronavirus pandemic has forced the FAI's hand.
Ireland faces Slovakia in the play-offs for the continental competition, but that tie is also subject to an indefinite delay.
McCarthy, whose contract was due to expire on 31 July, has therefore stepped aside early, with Republic Of Ireland Under-21 boss Kenny officially taking on the role earlier than his previous start date of 1 August.
"The Football Association of Ireland announces that Mick McCarthy is to be succeeded as national team manager by Stephen Kenny with immediate effect," a statement from FAI read.
"The handover has been agreed with both men in light of the delay to the European Championship play-offs caused by the COVID-19 pandemic.
"Mick McCarthy's contract was due to expire on July 31 after the UEFA Euro 2020 finals, with Stephen initially scheduled to step up from his Under-21 team role on August 1.
"This move allows Stephen Kenny time to plan for the European Championship play-off semi-final against Slovakia later in the year."