A garda who has alleged major criminal wrongdoing within the force has urgently sought clarification on how his allegations have been investigated, RTÉ has learned.

According to correspondence seen by RTÉ's This Week, Garda Nicky Keogh has written to Garda Headquarters, saying he is totally in the dark over how several related probes into his allegations are now being handled.

Garda Keogh first alleged in May 2014 that a garda detective was colluding with a drug dealer to supply narcotics into the midlands region.

Garda Keogh, who is a decorated officer, raised this with senior management. Since then, he claims that he was subject to bullying and harassment, and that his career has been effectively derailed.

Among the long list of investigations and reports, the conclusions of which he says have been denied to him, include his substantive allegation of criminality.

It was subsequently referred to the Garda Síochána Ombudsman Commission (GSOC) and since then he has had no clarity on what that probe found.

Secondly, in 2015, he complained that a related investigation which examined this alleged criminality was not properly carried out.

Assistant Commissioner Jack Nolan was appointed to head that inquiry up but since Mr Nolan retired earlier this year, Garda Keogh now has no idea who is in charge of that procedure, or if it is even ongoing, he says in his correspondence.

Speaking to RTÉ's This Week, Carrick-On-Shannon based solicitor John Gerard Cullen, who represents Garda Keogh, said that his client had no idea what was the current status of this complaint.

"We don't know whether it has been mislaid or lost. We don't have any timeframes. We don't have any reasons why it's being treated in this way or that way. We don't have any excuses for the delay," said Mr Cullen.

"No reasons have been adduced why we might have rational confidence, in circumstances where the delay will obviously prejudice the preservation of documents, and electronic evidence and it's self evident that justice delayed is justice denied."

Garda Keogh also outlines in the letter that on 27 March of this year, he then made a complaint of bullying and harassment in which he says he was singled out and subject to mistreatment after he brought the original complaint to senior management.

He says in the letter that the bullying complaint relates in part to a senior officer in the force who has sought promotion, and he has questioned whether the Policing Authority would have been made aware of this complaint.

On the bullying and harassment complaint, Mr Cullen said: "Garda Keogh invoked a formal procedure about six months ago, and we don't have any idea what has happened since then".

Finally, Garda Keogh also raised questions over a report requested by the former Justice Minister Frances Fitzgerald, and carried out by Garda HQ, into Garda Kehoe's treatment.

"Other than giving a very unusual sense to the word urgent, whereby one is left waiting for an answer for almost a year, nothing is known about the content of that investigation. The only thing we have been advised is that it has gone to GSOC," Mr Cullen said.

"Again the whole matter is inscrutable. We have no idea what's in that urgent report," the solicitor said.

Mr Cullen said that they had been denied even a redacted version of the report.

The Garda Press office said that as administrative offices were closed over the weekend, it was not possible for them to provide any detailed response to a series of questions from RTÉ, into the nature of the reports and investigations sought by Garda Keogh.