The GSOC has been investigating the relationship between some gardai and Co Louth drug dealer Kieran Boylan, who supplied gardai with information about other criminals, and against whom serious drugs charges were dropped allegedly because he was a garda informer.
A report on the corruption scandal is due to be published by the GSOC in the coming weeks and it is expected the matter will then be sent to the DPP, who will determine if gardai should be charged with criminal offences.
A senior source said: "Everything is pointing to the report being forwarded to the DPP; this has been a very lengthy and complex investigation, but there is light at the end of the tunnel now.
"Some of what went on is frankly shocking."
Yesterday Garda Commissioner Martin Callinan insisted that he and members of the force had given full assistance to the GSOC investigation.
"I have co-operated fully and facilitated fully all of the inquiries that the GSOC has requested. It would be unwise to go beyond that, given that the report will be available in a number of weeks," he said.
While accepting the investigation had taken four years, Mr Callinan said the delay was not attributable to any attempt by the Garda to frustrate the GSOC's work.
Boylan (37) escaped prosecution despite being caught with heroin and cocaine worth €1.7m in Ardee, Co Louth, by the Garda National Drugs Unit (GNDU) in October 2005.
At the time he was on bail for an earlier €1m drugs haul.
In custody, Boylan alluded to his involvement in entrapment operations and threatened to reveal all if charged.
The charges connected to the GNDU haul were dropped, then reinstated, but dropped again in 2008.
It was later alleged that Boylan had also obtained an international haulage licence from the Department of Transport after gardai provided misleading information to officials, which suggested he had no drugs convictions.
The licence was later revoked.
Gardai concluded their own inquiries into the case and decided that no prosecutions or disciplinary action should take place.