The Garda Síochána Ombudsman Commission has sent a file to the DPP on the case of Kieran Boylan, a convicted drug dealer against whom further drugs charges were dropped in July 2008.

The public interest inquiry has examined the nature of the gardaí's relationship with the drug dealer, whether or not he was permitted to continue dealing drugs while working as an informant, and whether a conviction based on information supplied by Boylan has been rendered unsafe.

It is the longest-ever inquiry conducted by the commission.

The DPP will now consider whether criminal charges should be preferred against members of the gardaí.

For the last four years, the commission has been investigating the circumstances surrounding the case and whether or not gardaí colluded with him in the movement and supply of drugs.

The convicted drug dealer faced six charges in connection with the seizure of €1.7m worth of cocaine and heroin at a transport yard in Co Louth in October 2005.

He was on bail at the time after he had been caught with €700,000 worth of cocaine and heroin in Dublin and Louth for which he was subsequently sentenced to five years in prison.

However on the last day of the court sittings in July 2008, the 2005 drugs charges against Boylan were dropped without explanation.

The Ombudsman's investigation into why those charges were dropped has also examined:

- The nature of Boylan's relationship with the gardaí

- If he was acting as an informant for gardaí, registered or not

- Whether any gardaí knew he was dealing drugs while acting as an informant

- If he supplied drugs to other drug dealers and then gave information about those drugs to the gardaí

- Whether or not a conviction secured on the basis of such information is now unsafe

The Ombudsman said there had been ongoing issues during the four-year investigation between GSOC and the gardaí in relation to the sharing of information, but would not comment on any specific case.

However, twice in the past month, the Garda Commissioner has denied any lack of co-operation.

The Ombudsman has also examined alleged threats against a Louth couple after they reported, in confidence to gardaí, drugs linked to Boylan, allegations that gardaí knew Boylan had a mobile phone in prison, which is against the law, and how Boylan managed to secure an International Road Haulage Operators Licence even though he was a convicted drug dealer.

The file of more than 600 pages is now with the DPP who will decide whether or not criminal charges will be preferred.