A police officer shot dead during a robbery in Ireland was dedicated, diligent and hugely popular, senior officers have said.

Detective Garda Adrian Donohoe, a father of two aged in his 40s, was gunned down just half a mile from home as he tried to stop raiders escaping with cash from a Credit Union in Co Louth.

Garda Commissioner Martin Callinan said: "He was a very popular member of An Garda Siochana, a very efficient member, someone who was revered both by his peers and members of the community where he served."

Mr Donohoe's widow Caroline, two brothers and members of his extended family are also members of the Gardai. Mr Donohoe was originally from Co Cavan but was actively involved with the community in Co Louth. He had two children, a boy and a girl, and was a keen GAA fan.

Detectives are still investigating whether subversive elements such as dissident republicans were involved in the murder or whether a criminal gang was responsible. Mr Callinan said: "There were up to four people involved in the actual raid. How many were involved beyond that will remain to be seen."

It is understood Mr Donohoe approached the raiders and a number of shots were fired before they fled in a grey Volkswagen Passat. It is thought he was hit in the head without warning and before he had drawn his garda-issue weapon.

A book of condolence has been opened at Dundalk garda station where the murdered officer was based. Former colleagues and members of the public have been attending the station with floral tributes and leaving messages of support for the Donohoe family.

Joe Crawley, 34, who lives a mile from the murder scene, said: "It is just shocking. I am angered, disgusted and saddened at the killing. I have never felt such an intense mixture of feelings. It was totally unnecessary."

A wide police cordon extending for several miles remains in place around the crime scene at Lordship Credit Union at Bellurgan, Jenkinstown which is about seven miles from the border town of Dundalk. Specialist uniformed technical officers could be seen scouring the hedgerows and bushes area for clues.

Irish President Michael D Higgins described the killing as a dreadful crime and said all Irish people would be truly appalled. Taoiseach Enda Kenny said the murder was an outrageous and appalling act of cold blooded violence.