Mr Carthy, 27, who suffered from bipolar depression, was killed on April 20, 2000, following a 25-hour stand-off at his family home.
He was shot four times by two members of the garda Emergency Response Unit (ERU), after he emerged from the house carrying a shotgun and ignored orders to drop the weapon.
The fourth and fatal shot was fired by Detective Garda Aidan McCabe, who said in evidence he believed Mr Carthy posed a threat to colleagues.
In his report published yesterday, Mr Justice Robert Barr found there was insufficient evidence to suggest Det Gda McCabe had acted unlawfully in shooting Mr Carthy.
But the shooting could have been avoided if the stand-off had been better managed by the officers in charge of the scene, Mr Justice Barr said.
He said the scene commanders — Superintendents Joseph Shelly and Michael Byrne — “had primary responsibility” for the circumstances which led to Mr Carthy’s death.
Of a series of errors they made, “the greatest mistake at Abbeylara was not preparing for an uncontrolled exit by Mr Carthy from his house, as actually happened”.
By failing to conceal officers in the vicinity of the house, they presented Mr Carthy with apparent targets, “thus causing Garda McCabe to shoot him with fatal consequences, in order to remove that risk”.
The judge discounted the possibility that Mr Carthy had been attempting “suicide by cop” — where a suicidal individual manipulates a situation so that the police have no choice but to use deadly force.
Mr Justice Barr urged the Minister for Justice and the Commissioner of An Garda Síochána to review garda command structures and revise training to ensure the mistakes at Abbeylara will not happen again.
The review should decide who takes overall command when the ERU is deployed to a scene — the unit commander or local superintendent.
A decision would also have to be made on whether local superintendents undergo “refresher training” as scene commanders for one week each year, and whether ERU officers would also benefit from such training.
Among other possibilities Mr Justice Barr suggested were increased training for garda negotiators, particularly for dealing with people affected by mental illness; basic instruction for garda recruits on mental illness; and the hiring of additional State psychologists to assist with siege-type situations.
He also suggested the adoption of a retraining model based around that of Victoria, Australia, which has been “highly acclaimed in other jurisdictions”.
Consideration should also be given to equipping the ERU with non-lethal weapons, such as Taser stun guns, he said, and the use of police dog teams at siege, and similar, situations.