At least 10 garda informers have been murdered over the past decade.

Fresh concerns have now been raised about the handling of what are known as covert human intelligence sources, or CHISs, following the brutal murder of Keith Ennis, whose remains were found in Amsterdam two weeks ago.

The 30-year-old Dubliner had turned garda informer after being found in possession of €16,000 in cash two years ago. A few days after the cash seizure, detectives visited his apartment in
Clondalkin where they discovered a Glock pistol and €2,000 worth of cocaine.

While in custody, he offered information on a major drugs distribution network in Dublin, which led to a €100,000 drugs seizure.

Ennis fled the country not long afterwards and little was known of his whereabouts until his dismembered body was found in a suitcase recovered from a canal in Holland.

Ennis is one of at least 10 known garda informers to have met a violent end in recent years.

Gardaí have defended how they handle confidential informants

"A Garda Liaison Officer attached to the liaison and protection section at Garda Headquarters, who is based in the Hague, is providing assistance to the Dutch police in relation to their investigation into the death of Ennis," a garda statement said.

"All individuals with information regarding the commission of crime are encouraged to give this information to An Garda Síochána. An Garda Síochána has a code of practice for the management and use of covert human intelligence sources, which reflects best international practice.

"The code of practice has been the organisational policy of An Garda Síochána since 3 April 2006 and sets out the professional standards required to maintain the confidence of the public and the courts in the use of informants.

"Individuals who meet specific criteria, as set out in the policy and procedures, are assessed and, where considered appropriate, are managed by garda personnel who have been specifically trained to internationally recognised standards in this area."

However, while there are safeguards to protect "touts", they often do not work and informers have frequently ended up dead after their criminal associates discovered they were talking to gardaí.

Other informants who have been killed in revenge for talking to gardaí include Declan Gavin, Christopher 'Git' McDonagh, Wayne Zambra and Paul Reay.

Gavin was stabbed to death outside a fast-food restaurant in Crumlin amidst suggestions that he had been passing information to gardaí.

The killing in August 2001 ultimately led to what has become known as the Crumlin feud, which has already claimed the lives of 12 men and sparked dozens of other assassination attempts.

Christopher 'Git' McDonagh was murdered in September of last year on the orders of a major criminal, who was already behind bars serving a sentence for manslaughter.

McDonagh was also suspected of informing, and despite warnings from gardaí about his personal security, was shot four times.

Wayne Zambra, another man murdered as part of the Crumlin feud, was also acting as a garda informer, sources said, while Paul Reay, who was shot in August 2006, met his end for the same reason.

"There are concerns that these men, sometimes in the very minor league of crime, are being utilised to bring in drugs or guns and then effectively thrown to the wolves," a senior garda source said.

Another notorious garda agent, Declan Griffin, was also murdered, while major gangsters including Martin 'Marlo' Hyland, Shane Coates, Martin 'The General' Cahill and Seamus 'Shavo' Hogan were all at one stage supplying information to detectives.

Martin 'The General' Cahill had also, when it suited him, passed gardaí information regarding the IRA, armed robberies and drug dealers. His former associate,
Séamus 'Shavo' Hogan, had his ears carved up to make him resemble a rat before he met his demise in a still unsolved killing in 2001.

Shane Coates, one of the notorious leaders of the Westies criminal gang, operated as a garda informer until he tipped off gardaí about an impending bank raid, only to rob another bank himself at the same time. He was murdered in Spain in 2004.

In 1995, the late Declan Griffin admitted to being an informant at his trial for trafficking heroin through Dublin airport.

Gardaí denied his claim but Griffin was cleared of the charges and went back to his life of crime. He was murdered in a pub in Dublin in 2003.