Co Louth, where Detective Garda Adrian Donohoe lost his life, was also where the last capital murder of a garda took place 35 years ago. 

On the morning of 27 June 1985, two men in their early 20s - Noel Callan and Michael McHugh - took part in an armed robbery outside the employment exchange in Ardee, Co Louth. 

In the ensuing chase, McHugh shot dead Garda Sergeant Patrick Morrissey at Rathbrist, Tallanstown. 

Both Callan and McHugh were convicted of the murder by the non-jury Special Criminal Court in December 1985. 

McHugh was initially sentenced to death for the murder. 

The sentence was commuted to one of 40 years' penal servitude by then President Patrick Hillery, seven days before the execution was scheduled to be carried out. 

Official State Papers from the time – released in 2015 - include documents from the Department of the Taoiseach that show how President Hillery "on the advice of the government … commuted the sentence of death passed on Michael McHugh to a sentence of penal servitude for 40 years". 

The government's decision to advise the President was "taken on the basis that the full sentence of 40 years will be served without remission". 

However, in 2013, Callan took a case to the Supreme Court, challenging the refusal of authorities to consider him for a reduction in time served. 

Callan's lawyers argued that several others jailed for capital murder whose death sentences were commuted to 40 years had been freed or got temporary release before the end of the 40 years. 

The Supreme Court ruled he was entitled to found he was entitled to be considered for 25% remission for good behaviour on his 40-year sentence. 

Callan was released in November 2015 and McHugh was released the following month, after serving 30 years in prison. 

A total of 89 gardaí have been killed in the line of duty. The most recent tragedy for the force was the fatal shooting of detective garda Colm Horkan in Castlerea, Co Roscommon.

See how RTÉ News reported the murder of Garda Sergeant Patrick Morrissey in 1985