Supt Patrick McMenamin described the victim as a “very respected, decent young man” from a “very respectable family”, who he said were “devastated”.
He declined to comment on lines of inquiry, but it is understood that the main suspect — a 15-year-old boy from a notorious criminal family — fled abroad within hours of the attack.
Garda Commissioner Nóirín O’Sullivan described the violence over Halloween — which also left two other young men fighting for their lives from stab wounds — as “horrific” and she said her thoughts were with the affected families.
Mr O’Reilly, also known as Lorcan Rooney, was fatally stabbed just before 2.45am on Sunday while attending Halloween celebrations at Oliver Bond flats, in Dublin’s south inner city.
He received a single knife wound to his chest, piercing his heart.
The stabbing took place in a large courtyard area surrounded by blocks in the complex. The deceased lived across the road from the flats in Robert Emmet Close.
It is thought around 20 people were present at the time, comprising separate groups of friends. Supt McMenamin, based at Kevin St Garda Station, urged those present between 2am and 3am to come forward.
“We do know that there are others that were present and know what happened or have information which could be vital to this investigation and I am appealing to those people to come forward to contact us here in the incident room at Kevin St, or anyone who was present, or has information.”
He said the Garda technical bureau had conducted a forensic examination at the scene, but he declined to say if weapons were recovered.
He assured those fearful of talking to gardaí, that their information would be treated in the “strictest confidence”, either by the team in the incident room at 01 6669400 or on the Garda Confidential Line 1800 666 111.
Detectives are trying to locate people who were at the Halloween celebrations.
However, they fear the suspected killer has fled abroad, possibly to Britain or Spain, where his father, a well-known gangland figure, has connections.
A friend of the deceased posted on Facebook that she could not believe what had happened: “I just keep thinking why him. He didn’t deserve it, such a harmless young fella, just wanted a good night.”
She said Lorcan, who she called by his nickname “Squeak”, died “because someone tried to be a hard man with a knife”.
Meanwhile, a 22-year-old man was was stabbed while attending a bonfire in Mulhuddart, northwest Dublin, on Halloween night remains in a critical condition. A 25-year-old stabbed in Clarehall, north Dublin, is also in a serious condition.