Veteran prop Cian Healy has also been left out after the 35-year-old suffered a calf problem in Saturday evening’s 17-13 warm-up win over Samoa.
He was helped from the field by medical staff in the first half at Stade Jean Dauger in Bayonne before being pictured on crutches.
Munster loosehead Jeremy Loughman has taken Healy’s place.
Connacht’s Cian Prendergast has been left out by Andy Farrell.
Leinster fly-half Ciaran Frawley and Munster hooker Diarmuid Barron also failed to make the cut.
Dan Sheehan, Ronan Kelleher, Jack Conan, Dave Kilcoyne and Keith Earls have been picked, despite recent injury issues.
Fly-half Johnny Sexton, who has now completed a three-match ban, will captain his country ahead of retirement.
Ireland begin their campaign on September 9 against Romania in Bordeaux and also face Tonga, world champions South Africa and Scotland in Pool B.
Aside from the sidelined Healy, wing Stockdale is the highest profile player overlooked by head coach Farrell.
The 27-year-old, Ireland’s joint-sixth highest try-scorer with 19, has become a peripheral figure since the last World Cup amid a series of fitness setbacks.
He started against Samoa due to Earls’ late withdrawal with a niggle but was taken off with a hamstring issue.
Veteran Earls and first-choice wide men James Lowe and Mack Hansen have been preferred to Stockdale.
Farrell’s decision to go with an 18/15 split of forwards and backs sees Ulster centre McCloskey picked and back-rower Prendergast, who started last weekend’s 29-10 win over England, overlooked.
McCloskey has impressed when selected but his opportunities at 12 have been limited by the presence of Bundee Aki and Robbie Henshaw, in addition to some untimely injuries.
The group includes 16 players who travelled to the 2019 World Cup in Japan under predecessor Joe Schmidt.
Sexton, Conor Murray and Earls are each set for their fourth World Cups.
Regular starters Hugo Keenan, Caelan Doris, Sheehan, Jamison Gibson-Park, Hansen and Lowe are among the 17 players preparing for their first.
Versatile back Jimmy O’Brien, fly-half Jack Crowley and lock Joe McCarthy have each made the grade, having only made international debuts in the autumn.
Forwards: Ryan Baird (Leinster), Finlay Bealham (Connacht), Tadhg Beirne (Munster), Jack Conan (Leinster), Caelan Doris (Leinster), Tadhg Furlong (Leinster), Iain Henderson (Ulster), Rob Herring (Ulster), Ronan Kelleher (Leinster), Dave Kilcoyne (Munster), Jeremy Loughman (Munster), Joe McCarthy (Leinster), Peter O’Mahony (Munster), Tom O’Toole (Ulster), Andrew Porter, (Leinster), James Ryan (Leinster), Dan Sheehan (Leinster), Josh van der Flier (Leinster).
Backs: Bundee Aki (Connacht), Ross Byrne (Leinster), Craig Casey (Munster), Jack Crowley (Munster), Keith Earls (Munster), Jamison Gibson-Park (Leinster), Mack Hansen (Connacht), Robbie Henshaw (Leinster), Hugo Keenan (Leinster), James Lowe (Leinster), Stuart McCloskey (Ulster), Conor Murray (Munster), Jimmy O’Brien (Leinster), Garry Ringrose (Leinster), Johnny Sexton (Leinster, captain).