Gardai have not established a definite motive for the attack and the victim is not understood to be involved in organised crime.
He is a man aged in his late 30s who is understood to be homeless but originally from the Coolock area.
He was discovered outside Belcamp College after suffering a major loss of blood and rushed to hospital where his condition is described as serious.
Gardai have appealed for information in the mysterious case.
“Gardaí are investigating the discharge of a firearm that occurred yesterday evening, 11th August 2021 in the Coolock area of Dublin,” a garda spokesman said.
“The incident is believed to have occurred shortly after 9pm.
“A man, aged in his 30s, was later taken to Beaumont Hospital where he is currently receiving treatment for serious injuries.
“A technical examination was carried out at the scene earlier this morning, 12th August 2021. No arrests have been made and investigations are ongoing.
“Gardaí are appealing to any persons who may have information and who were in the Belcamp area between 9pm and 10pm to contact Coolock Garda Station on 01 666 4200, the Garda Confidential Line 1800 666 111 or any Garda station,” he explained.
The Oblate fathers founded a college at Belcamp in 1893 and opened a boys' boarding school there.
The school closed in 2004, the year it was acquired by developer Gerry Gannon. As the building lay derelict in the subsequent years, it was subjected to a number of arson attacks.
It went up in flames twice in two days in 2016 and in May, 2020, firefighters attended another blaze at the site.