He was brought to City by former manager Sven Goran Eriksson but has suffered injury problems since returning to Manchester, playing just one match this season in the League Cup.
"We wanted to try and add different options to our attacking threat and John will certainly do that," Stoke manager Mark Hughes told the club's website.
"He's someone who if you get him in the right areas will make the most of the opportunities that come his way because he's a natural goal scorer," Hughes added.
Former Manchester City player Ireland, 27, has been on loan at the Britannia Stadium this season, making 16 appearances and scoring three goals.
"Stephen is a talented footballer and has shown what he's capable of since he joined us," Hughes said. "We're delighted to have made his switch a permanent arrangement."
(Reporting by Mark Young, editing by Ed Osmond) nL3N0KO456
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