One man, in his 40s, has been arrested in connection with this investigation and is currently detained in Castlerea Garda Station.
Garda Commission Drew Harris said: “I am deeply saddened by the death of my colleague Detective Garda Colm Horkan.
“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 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.
“The fatal shooting of Detective Garda Colm Horkan remains under investigation. I want to appeal to anyone with information on this incident or witnessed this incident to contact Castlerea Garda Station at 094 962 1635.”
A spokesman said: “It is with deepest sadness An Garda Síochána confirms the death of our colleague, resulting from fatal gunshot wounds received during an incident in Castlerea shortly before midnight on Wednesday 17th June 2020.”
President of the Garda Representative Association, Jim Mulligan paid tribute to the “greatly respected” and experience garda.
He 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.
“He is survived by his father, sister and four brothers.
“Our thoughts are with his family and colleagues at this very distressing time.
“Ar dheis Dé go raibh a anam.”