Earls missed Ireland's summer tour to New Zealand and Australia with a groin injury and when that had cleared up, he then damaged an ankle in training with Munster in Cork in August.
Commenting on the Limerick youngster's return, Munster squad advisor Mick Galwey said: "It's great to have Keith back in the squad, He did a full session on Tuesday and came through that with no ill-effects.
"The only concern would of course be match time. He hasn't had any of that for a long time.
"But it's great to have him back in the matchday squad, particularly when you think of the injuries we have. It's important to get back as many players as we can.
"A few weeks back we weren't sure when exactly he'd be back, whether it would be Heineken Cup round 1 or 2. He's in there now and that's a huge boost."
Earls is one of seven changes - two of them positional - that Munster coach Tony McGahan has made to the team for Saturday's Pool 3 clash with London Irish in Reading.
With Lifeimi Mafi suspended, Earls joins Sam Tuitupou in Munster's midfield for the first time.
Johne Murphy, who featured at centre for the opening three Magners League matches, has been named at full-back ahead of Paul Warwick who drops to the replacements.
Illness ruled Tony Buckley out of the league defeat to Leinster, but he returns to the tighthead position this weekend and there is a major shake-up in the Munster back row - Denis Leamy skippers the side from blindside flanker and James Coughlan makes his second start of the season at number 8.
New Zealander Tuitupou is set to make his Heineken Cup bow, having played for Worcester Warriors before in the Amlin Challenge Cup.
It will be a first Heineken Cup start for both hooker Damien Varley and lock Donnacha Ryan, while replacements Mike Sherry and Duncan Williams are standing by to make their debuts at this level.
Galwey added: "Of course, everyone was disappointed with the result (against Leinster). But we'd set our goals for the start of the season in the Magners League and we're top of the table at the end of that phase.
"If we'd been offered that at the outset, would we have taken it? Of course we would.
"There are no trophies won in October. Losing to Leinster is not the ideal preparation. We thought the away win in Glasgow was the the ideal preparation for Leinster. A bonus point win should have been ideal preparation but it wasn't.
"Sometimes when you're winning all the time you don't learn a whole lot about yourselves. When you lose a game like that, you know you have to go back to the drawing board. And that's what we've done this week."
MUNSTER (v London Irish): Johne Murphy; Doug Howlett, Keith Earls, Sam Tuitupou, Denis Hurley; Ronan O'Gara, Peter Stringer; Wian du Preez, Damien Varley, Tony Buckley, Donncha O'Callaghan, Donnacha Ryan, Denis Leamy (capt), Niall Ronan James Coughlan.
Replacements: Mike Sherry, Marcus Horan, John Hayes, Mick O'Driscoll, David Wallace, Duncan Williams, Paul Warwick, Scott Deasy.