The man, who is understood to have been living in the tent, was later pronounced dead in hospital.
He had serious injuries consistent with an assault.
The victim was found unconscious a short distance from the tent shortly before 1am in an area increasingly used by homeless people in recent months.
Garda Superintendent Michael Comyns confirmed detectives had launched a homicide investigation.
“We are appealing to those who were in Mardyke Walk area between last night and 1am this morning who may have witnessed any activity in the area to come forward,” he said.
“We are also appealing to motorists who may have passed through Mardyke Walk around these times to make dash-cam footage available to us.
“There are a number of guest houses in the area with visitors to the city staying last night and we would ask that if anybody seen or heard anything, no matter how insignificant they think it is, to contact us.
“We are currently treating this as a homicide investigation.”
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