Colleagues of tragic detective Colm Horkan rushed to his aid after hearing gunshots from the nearby garda station.
The experienced garda was on patrol in Castlerea, Co Roscommon when he stopped a man driving a moped shortly after 11.45pm on Wednesday.
It's understood Det Horkan got into a struggle with the man who managed to get hold of the garda's official gun before firing multiple fatal blows.
It's believed officers inside Castlerea Garda Station heard shots fire and ran out to find Mr Horkan lying on the ground on Main Street with several gunshot wounds.
Speaking to the media, Garda Commissioner Drew Harris said: "Detective Garda Horkan and a member of the public got in an altercation during which the service pistol the garda was carrying was used against him and he was shot and fatally wounded.
"He received very prompt and professional medical attention but he died from his wounds."
A murder investigation is now under way.
A suspect, in his 40s, has since been arrested in connection with the probe and is being detained at Castlerea Garda Station.
Det Horkan, 48, from Charlestown, Co Mayo, had been a member of An Garda Siochana for more than 24 years.
The Garda Representative Association has described the big sports fan as a "highly respected" and experienced member of the force.
GRA President Jim Mulligan said: "On behalf of the GRA, I extend our deepest sympathy to the family of our colleague fatally shot in the line of duty in Castlerea, Co Roscommon overnight.
"Our brave colleague died as result of injuries sustained when he was responding to an incident. He was based in Ballaghaderreen, Co Roscommon, in the Castlerea District.
"A native of Co Mayo, he joined An Garda Síochána in 1995, and was an experienced detective greatly respected by colleagues.
"Our thoughts are with his family and colleagues at this very distressing time."
Garda Commission Drew Harris said he is" deeply saddened" by the death of Mr Horkan.
In a statement released on Thursday morning, added: "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.
"Detective Garda Colm Horkan is the 89th member of An Garda Síochána killed in the line of duty.
"Today is a terrible reminder of the significant sacrifices, including the ultimate sacrifice, that Gardaí make to keep people safe."
