He was sentenced on November 7 last year after a trial which saw one colleague jailed for three years with 18 months suspended and another given a four-month suspended sentence.
With remission he should have been held until early August.
But sources revealed that he was released last week, two and a half months early.
However, the source added that many of the prisoners in the open facility get up to four months off their term.
The former officer lost his job as a result of his actions which diverted possible camera evidence away from his colleagues, Daniel Hickey (29)and Martha McEnery (42).
Sources said that John Burke had served time in the segregation unit at the Midlands in Portlaoise before being transferred to the open prison about three months ago.
At their trial Judge Leonie Reynolds condemned their actions as "obscene acts of criminality" and said their careers were "in tatters".
She said they were fortunate that the injuries inflicted against Anthony Holness (38), on January 29, 2010, were not "more serious".
It was the first case to be brought by the Garda Ombudsman's Office before a jury.
The judge said Burke's actions in moving the camera brought a "disturbing and sinister dimension of this case".
It was an ill-fated decision that had "life-changing consequences".
Burke, is married with one child, and had 16 years' service as a garda but later secured a job with an engineering firm.