The murder of a detective during a credit union hold-up has been branded an attack on the state and its police force.
He was shot in the head on Friday night by a gang as they stole 4,000 euro at a rural credit union.
"I see what occurred effectively as an attack not just on an individual garda but on all members of the force, essentially an attack on the state and this will not be tolerated," the minister said.
Mr Shatter was briefed by local detectives in Dundalk and met Garda Joe Ryan, who was held at gunpoint after seeing his colleague murdered in cold blood.
"I had the opportunity to meet Joe Ryan who was the other Garda Detective involved in this event and I expressed my good wishes to him," the minister said.
"I think we have to admire his courage. He's obviously still upset. I think everyone in the station is understandably upset but I think they're very focused at bringing to justice those who are responsible for the barbaric and callous murder that took place last Friday evening."
Earlier, Mr Shatter said the gang must have known gardai would have been escorting takings from the Lordship Credit Union in rural Co Louth on Friday night.
Police suspect the same cross-border criminal gang which stole 62,000 euro from the credit union more than a year ago may have been behind Mr Donohoe's murder.
A burnt out car found near a forest in the Darkley area of south Armagh near the town of Keady was being forensically examined by police in Northern Ireland.