Detective Garda Colm Horkan is the 89th member of An Garda Síochána to lose his life in the line of duty.
The 49-year-old Airport Road, Charlestown native was shot several times and killed by his own gun after a scuffle with another man in the centre of Castlerea, where he was stationed, close to midnight on Wednesday night (17th).
Up to 15 shots were discharged from his gun.
Garda Horkan had been a member of the force for 26 years having also served in Ballaghderreen.
When news of his shocking death filtered, sympathies were extended to his family, friends and colleagues.
President Michael D Higgins said he was shocked to hear of the death of a Detective Garda while on duty.
He said: "News of the tragic death of a Garda detective on duty in Castlerea has come as a shock to us all.
"An Garda Síochána play a crucial role in our communities and this loss of life damages our society as a whole.
"It is appalling what is alleged to have happened.
"Of course it's such a blow to Colm Horkan's family and his colleagues and his friends and the death of a garda, active on duty, is a blow to us all.
"My sympathy goes out to the incredible loss his family will have suffered, but also to those who have worked with him and bearing in mind also that he is somebody who has given more than 20 years service to the keeping of peace in the interest of all of the people.
"The death of a garda is a blow to the whole community and every member in it."
Taoiseach Leo Varadkar also expressed deepest sympathies to Garda Horkan's family and friends.
He said: "Every day our Gardaí put themselves on the frontline of crime prevention, on behalf of all of us.
"This requires regular acts of bravery and courage. Sometimes the outcome is tragic and a Garda makes the ultimate sacrifice in the course of their duties."
Garda Commissioner Drew Harris said Garda Horkan has served his country with distinction and pride during his 26 years of service. "I am deeply saddened by the death of my colleague," he said.
"My thoughts, and the thoughts of all personnel in An Garda Síochána, are with the family, friends and colleagues of Detective Garda Colm Horkan at Castlerea Garda Station and the Detective Unit he served in.
"Colm served the communities he worked in with distinction and pride during his 24 years as a member of An Garda Síochána.
"It is a terrible reminder of the significant sacrifices, including the ultimate sacrifice, that Gardaí make to keep people save.
"I want to thank the local community in Castlerea and the country as a whole for the support they have shown to An Garda Síochána and individual Gardaí even in the short time since the incident," he added.
Minister for Justice Charlie Flanagan also expressed his shock and sadness at the death of Garda Horkan.
"I want to express my heartfelt sympathy to the family of the brave Garda who died doing his duty serving his community and to the wider family of An Garda Síochána who will be heartbroken.
"Our gardaí work to keep Ireland safe at all times and this is a very difficult day for all of them and their families," he said.
Local TDs from the Sligo/Leitrim constituency also expressed their deep shock and sadness at the untimely passing of Detective Garda Horkan.
Fine Gael TD Frank Feighan described his death as a brutal and sad tragedy.
He said: The fatal shooting of a member of An Garda Síochána is a brutal and sad tragedy for the officer's family and his colleagues in the force."
He said his condolences were with the family, relations, friends and work colleagues of Garda Horkan. "It a sharp reminder to the heart of the nation of the danger that people who serve this nation put themselves in every single hour of every day. It is a brutal and sad tragedy. We offer our support to all members of An Garda Síochána at this time. They protect the Irish State and Irish people at all times. And that role has resulted in the loss of another member of the force.
"Our Guardians of the Peace live and work among us. They are members of every community in every county - not least in Castlerea, County Roscommon. Their work can never be taken for granted by the Irish people. The loss in An Garda Síochána is immeasurable today. All our thoughts and prayers are with the Detective Garda, his family and those he worked with," Deputy Feighan said. Fianna Fáil TD for Sligo/Leitrim Marc MacSharry said Detective Horkan's death is a stark reminder of the dangers all gardaí face on a daily basis while keeping the public safe and extended condolences to Detective Garda Horkan's family, friends and the wider community. Independent TD for Sligo/Leitrim Marian Harkin said the death of Garda Horkan is a sharp reminder of how the Gardaí put their lives at risk to protect society.
She said: "Such shocking and heartbreaking news from Castlerea for the family and friends of Detective Garda Colm Horkan.
"For the rest of us a sharp reminder that Gardai put their lives on the line to protect all of us. Ar dheis De go raibh a anam dilis."
Sinn Fein TD Martin Kenny said it was a shocking incident and a difficult day for An Garda Síochána.
He expressed his deepest condolences to the family and colleagues of Detective Horkan and said he was shocked to hear the news of the shooting dead of a member of An Garda Síochána in Castlerea.
"I want to send my deepest condolences to the family and colleagues of the Detective Garda who has been killed in the course of his duties in Castlerea in County Roscommon last night. This is a truly shocking incident.
"This is a reminder of the risks that Gardaí take every day in the line of duty.
"This is difficult day for An Garda Síochána, the Garda's family and his friends and all of his colleagues.
"My thoughts are with them all at this incredibly difficult time."
Minister for Health Simon Harris said: "Shocking and devastating news this morning from Castlerea. Thoughts and prayers with the family, friends and colleagues of the Garda. An attack on a Garda is an attack on all of us. Every day they work to keep us all safe."
Books of Condolences were opened in Garda stations across the country, including in Sligo, Ballymote and Carrick-on-Shannon for people to sympathise with the Horkan family.
It is intended that the Books of Condolence from Garda stations around the country will be presented to family of the late Detective Garda Colm Horkan.
Det. Garda Horkan, who was on his own on anti-crime patrol had stopped to talk to a man on Main Street, Castlerea when the incident escalated. Gardaí had received a number of calls shortly before 11pm reporting that a local man, known to gardaí, was driving a motorbike in an erratic manner up and down the Main Street of the town without lights. It is understood there was no initial threat to the Garda when he pulled over to speak with him after getting out of his unmarked patrol car.
However, a tussle ensued and the man grabbed the Garda's SIG Sauer P226 firearm, which does not have a safety catch, and fired the handgun wildly with all 15 rounds being used, up to five being fired at Garda Horkan.
Two uniformed Gardaí subsequently arrested the man at the scene.
Garda scene-of-crime examiners spent much of Thursday extracting spent rounds from surrounding walls and buildings.
He had been appointed to plain-clothes duty a few weeks previously, having spent most of his 26-year career in uniform.
Garda Commissioner Drew Harris later described the murder as a random act and added: "This is tragic and it is an attack upon us all. Policing is an extension of good citizenship." After graduating from the Garda College in Templemore, Garda Horkan spent some years in Finglas in west Dublin. He served in Castlerea and Ballaghaderreen for the other 21 years. Detective Garda Horkan was predeceased by his mother Dolores and twin sister, Colette. He is survived by his heartbroken father Marty, sister Deirdre, brothers Aidan, Brendan, Dermot and Padraig, sisters-in-law, aunts Kitty (Walsh), Charlestown, Colette (Brennan) and Vera (Horan) USA, nieces, nephews, relatives, colleagues in An Garda Síochána, neighbours and a wide circle of friends.