Comm Callinan was due to retire next August -- but will now continue to oversee a number of controversial changes including station shutdowns and roster changes.
A ban on gardai serving beyond the age of 60 has had to be adapted to allow the Commissioner remain in the force, and the move was rubber-stamped by the Cabinet.
"At a time of such significant organisational reform, I believe that it is desirable that there should be continuity in Garda leadership, and I am delighted that Martin has agreed to this extension," said Minister Shatter.
He noted that a new garda roster was now being piloted, the station networks were being reformed and modernised, and an effective policing service was being maintained at a time of significant resource constraint.
The closure of up to 80 of the country's 700 garda stations will be seen as one of Commissioner Callinan's biggest tests.