Customers in a pub fled for safety as a gunman opened fire on a convicted criminal, gardai said.
Paul Cullen was gunned down in front of his brother and wheelchair-bound father in the Cabra House pub, close to his home in Dublin's northside.
Superintendent David Taylor described the capital's latest gangland-style attack as a callous and brutal murder of a young man in a public place.
"There was a number of people in the pub at the time and, as you can imagine, when we got there it was a pretty chaotic scene and people were obviously fleeing for their own safety," Mr Taylor continued.
"So we are asking anyone who may have been in the pub at the time, or in the hours beforehand, to come forward."
The senior garda revealed members of the Cullen family were among about 20 customers in the bar on Fassaugh Avenue, off the Navan Road, and were traumatised by the shooting shortly after 7pm on Sunday night.
A masked man fired up to six shots at the 27-year-old - who was a former associate of murdered gangland boss Eamon Dunne - while a second masked man waited outside on a dark motorbike.
"We have a family liaison officer appointed and dealing with the family," he said.
"It was pretty traumatic to witness such a callous and brutal murder."
Deputy State Pathologist Khalid Jabbar is carrying out a post-mortem examination while forensic officers continue to examine the scene, where flowers have been left. The card reads "you'll never walk alone".