The scene at North Strand Road Dublin this morning.
Pic: Justin Farrelly.
He was rushed to St James's Hospital last night but was pronounced dead earlier today.
A post mortem is due to take place soon.
The four men that were arrested in connection to this investigation remain in custody.
Gardai have this evening renewed their appeal for information about the attack which happened at Cow's Lane in Temple Bar at 8.45pm.
It has emerged that after Mr O'Connor was struck, the 33-year old man who allegedly punched him was attacked himself and knocked unconscious.
The suspect from the Hartstown area of the northside was brought to St James's Hospital for treatment to severe facial injuries which may include fractures.
However after being treated in the hospital, he was arrested at 4.15am and is currently being held in Irishtown Garda Station. He is well known to gardai for previous incidents.
Another two men, aged 35 and 27, who are suspected of assaulting Mr O'Connor's attacker were also arrested and are today being questioned in Pearse Street Garda Station. They are from the Lucan area.
A fourth man from Killester who is an associate of the man who attacked Mr O' Connor was also arrested and he is being quizzed in Kevin Street Garda Station.
Gardai say a number of people in the vicinity took photographs and video as the incident escalated.
"The attack was pretty brutal and people may have been recording it because they felt afraid to intervene," said one source.
"But any footage would help the investigation and gardai would be anxious to see it," they added.
Investigating Gardai wish to renew their appeal to anybody who may have witnessed the incident or were in the vicinity of Cows Lane between 8.30pm and 9pm, or it is believed that some witnesses took images of this incident as it occurred and Gardai are appealing to all persons to contact them at Pearse Street Garda Station at 01 - 6669000, The Garda Confidential Line 1800 666 111 or any Garda Station.