Widespread condemnation has followed the killing of Det Garda Adrian Donohoe who is the first officer to be shot dead on duty since 1996.
President Michael D Higgins described the killing as a dreadful crime and said all Irish people would be appalled.
Taoiseach Enda Kenny said "this was an outrageous act of cold-blooded violence that has left a family without a husband and father and the Garda Síochána without a brave and valued member".
Minister for Justice Alan Shatter said no effort would be spared in bringing those responsible for the murder to justice.
"The Irish people rightly have great respect and admiration for members of An Garda Síochána and the work they do in protecting the community.
"They will share my revulsion and horror that a garda has so tragically lost his life in the course of his duties. Our thoughts and prayers are with the family, friends and colleagues of Adrian, who lost his life in carrying out his duty in the best traditions of the brave men and women of An Garda Síochána."
Tánaiste Eamon Gilmore said he was "appalled and deeply saddened".
Sinn Féin president Gerry Adams expressed his shock at the crime.
"I extend my deepest sympathies to his family and colleagues and appeal to those with information about this crime to co-operate with the gardaí."
'Assassination'
Minister for Communications Pat Rabbitte said the shooting did not happen because of the closure of Garda stations or cuts in the budget. It was a "cold-blooded assassination" that demonstrated how gardaí put their lives at risk every day.
Fianna Fáil leader Micheál Martin said "like the murder of Garda Jerry McCabe, last night is a horrible reminder that the gardaí stand apart. Their role is different to that of every other public servant and we should never forget that."
Fine Gael TD Peter Fitzpatrick, a former Louth football team manager, who played football with Det Garda Donohoe, described him as a "gentle giant whose wife and two children meant the world to him".
Fianna Fáil Cavan Monaghan TD Brendan Smith said the slain officer was well known and respected in his home town of Kilnaleck, Co Cavan.
Criminal thugs
Fianna Fáil justice spokesman Niall Collins said it was necessary to send the strongest possible message to "these criminal thugs that they are not untouchable. An attack on an on-duty garda, who is working to keep our communities safe, is a direct attack on the public itself."
Sinn Féin justice spokesman Pádraig Mac Lochlainn said the killing was yet another awful reminder of the immense risks "our gardaí take every day to protect our communities".
Fianna Fáil Louth TD Séamus Kirk said the killing was an "attack on civilised society".
Chairman of the Oireachtas justice committee David Stanton said "every effort will be made to bring these killers to justice". Labour Louth TD Jed Nash described the killing as a "despicable and evil act perpetrated by vile criminals".
Fianna Fáil Meath East Senator Thomas Byrne said it was a brutal murder of a man who "was gunned down while trying to protect others".
Chairman of Louth County Council Cllr Finnan McCoy said it was a black day when a man carrying out his duty on behalf "of us all is taken in such a cruel and callous way".
Fianna Fáil Cllr Declan Breathnach said there were those who knew the perpetrators and they should make a call to the Garda confidential phone line.
Sinn Féin Louth county councillors Jim Loughran and Edel Corrigan said "there is no excuse for the murder that took place in our community".
