Police say married father-of-two Sean Fox was shot multiple times by two gunmen in west Belfast on Sunday.
Detectives from the PSNI's Major Investigation Team investigating the murder of the 42-year-old at Donegal Celtic Sports and Social Club have released CCTV footage along with details of a reward from Crimestoppers.
In a statement, police added that the gunmen "were inside the premises for approximately 21 seconds" and "fired 20 bullets".
Crimestoppers, who are independent of the police, has offered a reward of up to £20,000 for anonymous information it receives that leads to the arrest and conviction of those responsible for the murder.
Detective Superintendent Eamonn Corrigan said: "On Sunday afternoon, at around 2.25pm, two masked gunmen entered the Donegal Celtic Sports and Social Club on the Suffolk Road in west Belfast, and made their way past several people.
"Sean Fox, who was a married father-of two and a regular visitor to the club, was singled out as he sat enjoying a drink in a busy function room.
"He was shot multiple times by both gunmen. At one point, both gunmen stood over Sean, shooting him, as he collapsed to the floor.
"This was a calculated, planned, ruthless execution. It was carried out in broad daylight and in the presence of others.
"The gunmen arrived, carried out the murder and left the club in under a minute. They were inside the premises for approximately 21 seconds. They left on foot, along the Suffolk Road, in the direction of Gweedore Gardens.
"This is a popular local club widely used by local families with young children. The gunmen fired 20 bullets and we have identified multiple strike marks. Anyone in that room or indeed nearby, could have been killed or seriously injured on Sunday.
"This cold-blooded act has taken a man's life. It has left a family bereft, and many others in the local community shocked. This barbaric act has no place in any society."
Police are asking anyone with information to get in touch.
Detective Superintendent Corrigan added: "Were you in the social club on Sunday afternoon? Did you witness the incident, or see the two masked men arrive or leave the club? Were you in the surrounding area at the time? If you have any information, or if you have any mobile or dash cam footage, please contact us on 101.
"One suspect is a man of athletic build. He was wearing a blue hooded-top, a peaked baseball cap, dark face covering, two-toned gloves (pale on top and dark on the bottom), dark-coloured bottoms and dark shoes/trainers.
"The second suspect is also a man of athletic build, and was wearing a distinctive light (possibly grey) hooded-top with three red lines down each arm, a peaked baseball cap, a light-green face covering, two-toned gloves (pale on top and dark on the bottom), dark-coloured bottoms and dark shoes/trainers.
"If anyone wants to stay anonymous, they can give information to the independent charity Crimestoppers on freephone 0800 555 111 or online at www.crimestoppers-uk.org. There is no caller line display, no 1471 facility and computer IP addresses are never traced or provided to detectives."
The PSNI added that additionally, information can be provided to police through the Major Incident Public Portal Link: https://mipp.police.uk/operation/PSNI22R23-P01.