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An Garda Síochána
The garda (Irish police) officer who was shot dead in County Roscommon on Wednesday was Detective Garda Colm Horkan.
An Garda Síochána said the incident happened in Castlerea shortly before midnight.
It is understood the shooting took place near a police station in Main Street in the town when the detective's firearm was taken from him.
A man in his 40s was arrested a short time later.
In a statement, An Garda Síochána said it confirmed the death "with deepest sadness".
The scene of the shooting has been preserved for forensic examination and the State Pathologist's Office has been informed.
'Shock to us all'
The Taoiseach (Irish prime minister) extended his deepest sympathies to the family and friends of the garda officer.
"Our thoughts today are with all those grieving as a result of this tragic incident," Mr Varadkar said.
Irish president Michael D Higgins said the news of the tragic death came as a "shock to us all".
In a statement, President Higgins said that "An Garda Síochána play a crucial role in our communities and this loss of life damages society as a whole".
Sinn Féin president Mary Lou McDonald said it was "a shocking incident" and "a sad and difficult day for not just An Garda Síochána in Roscommon but across the state".
Irish Justice Minister Charlie Flanagan described the incident as "tragic" and said a murder investigation is underway.
"My heartfelt sympathy to his family, friends and all Gardai," he said.
PSNI Chief Constable Simon Byrne added his sympathies and said the death was "awful news".
He said: "Thoughts from all PSNI with his family, friends and colleagues at such a difficult time."