No criminal charges are likely to be brought in the case of a senior garda detective who is alleged to have had an improper relationship with a convicted drug dealer, the Sunday Tribune understands.

RTE's Prime Time alleged last week that the garda conspired to have trafficking charges against a Co Louth drug dealer dropped.

Prime Time was repeating claims made over the past year by a Sunday newspaper against the senior officer which resulted in the Garda Síochána Ombudsman Commission (GSOC) and the gardaí launching separate inquiries.

However, senior garda sources say no evidence has been found to back up the claims, meaning it is highly unlikely that charges will ever be filed against him. Both probes have been going on for nearly five months.

Last July, on the day the courts recessed for summer, it was announced that the case against 38-year-old Kieran Boylan, for possession of €1.7m worth of heroin and cocaine in October 2005, was being struck out.

A barrister for the DPP said the decision had been taken at the highest level but offered no other information.

There have been allegations that the top tier of garda management was responsible because Boylan, who is from Ardee, threatened to reveal his relationship with senior members of the force if he was prosecuted.

Boylan has alleged that he was a garda informant and had participated in entrapment operations, where gardaí delivered drugs he supplied to petty criminals who were later arrested.

He has previously bragged that he was a garda agent who was untouchable by the law.

Boylan has named members of the gardaí with whom he was allegedly working in tandem, and the two investigations have been examining this relationship to see if Boylan was being allowed to continue to deal drugs in return for giving up other dealers.

A senior garda source said: "The gardaí and the ombudsman have both been looking into this for months and there is nothing criminal that has arisen. There have been a lot of rumours and allegations about this officer but they have not stood up. A lot of this is coming down to rivalries within the gardaí and certain journalists have been fed information and have printed serious allegations which amount to little more than one side of a story.

"To put it quite simply there is no smoking gun and it is hard to see any criminal charges ever being filed in this case despite the motivation of a number of different parties."

The garda accused of wrongdoing was attached to a national crime unit investigating the activities of serious criminal gangs and drug dealers. He is regarded in the force as an extremely effective officer who might use unorthodox methods.

It is understood he was interviewed under caution by GSOC detectives. A GSOC spokesman declined to comment on its investigation.