Head coach Andy Farrell said Ulster back Stockdale would “not be available to us” as he named a 36-man selection for the tournament.
Fly-half Johnny Sexton will captain his country for a second successive Championship campaign, fitness permitting. Sexton was was taken off as a “precautionary” measure during the second-half of Leinster’s Pro14 victory over Munster on Saturday amid concern about the recurrence of his hamstring injury.
Iain Henderson, Tadhg Furlong, Peter O’Mahony and James Lowe were also all selected, despite injury concerns.
Ireland begin the tournament away to Wales on Sunday, February 7.
Munster scrum-half Casey, who won a Grand Slam with Ireland Under-20s in 2019, trained with the squad last October, while Ulster prop O’Toole was named in his country’s squad at the start of the 2020 Championship but did not feature.
Experienced prop Furlong has not played for almost a year due to back and calf problems.
Ulster captain Henderson has been sidelined since suffering knee ligament damage in the Autumn Nations Cup win over Scotland early last month, while Leinster wing Lowe has recently returned to training following a groin problem and flanker O’Mahony suffered neck and leg issues playing for Munster at the weekend.
Coach Farrell told the IRFU website: “Since the Autumn international window the club season has had a good few disruptions and the cancellation of the European fixtures means there have been less opportunities for players to show form or to return from injury and get some minutes under their belts.
“Tadhg (Furlong) hasn’t played in quite a while but he is ready to join the group.
“Unfortunately Jacob (Stockdale) will not be available to us.”
Ireland play Scotland in the fourth round of fixtures, at BT Murrayfield on Sunday, March 14.
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