The victim was punched or slapped by one of the men, causing him to fall onto the road and under a passing Dublin Bus.
He suffered “catastrophic” injuries.
The tragedy happened in front of a large number of people at a busy junction in Dublin City Centre during rush-hour on Thursday.
The appalling nature of the injuries prevented the identification of the man, and he was not carrying ID.
It is understood the postmortem examination, conducted yesterday, has provided gardaí with a number of lines of inquiry.
It is thought the deceased may have had a body piercing and officers are using that to try and trace his identity through hostels, homeless agencies, and emergency wards.
Gardaí have put the man’s fingerprints through their systems, but no matches have come up. Sources said this was fairly unusual as most homeless people would have had contact with gardaí at some stage.
The investigation so far indicates that the deceased, thought to be in his 20s, was begging outside a shop on Nassau St, near the junction with Dawson St, at about 5.40pm.
Two other homeless people are thought to have approached him and hassled him for change.
The man is thought to have refused to hand over the money and tried to flee. He went towards the corner of Nassau St and Dawson St. At some stage one of the men slapped or punched the victim and he stumbled, falling or slipping off the footpath.
He went under the number 145 double-decker bus, in front a large number of shocked eye-witnesses.
The junction is very busy and the end of Dawson St narrows as it curves onto Nassau St, typically with pedestrians right up against the kerb.
Gardaí have collected CCTV footage in the area.
By lunchtime yesterday they tracked down one of the two men involved and arrested the 29-year-old under Section 4 of the Criminal Justice Act.
He is not thought to have cooperated with gardaí in identifying the victim.
Gardaí were closing in on the second man yesterday and were hopeful of making an arrest soon.
“This is an awful incident,” said a Garda source. “The poor man suffered catastrophic injuries all over a shake down, something mundane.”
Gardaí said there was no evidence the attacker intended to push the victim onto the road and that it was “a knock-on effect” of a slap or a punch.
The bus driver is said to be deeply traumatised by the tragedy and is trying to help both the gardaí and Dublin Bus in their separate investigations.