Alan Hatch, aged 38, hit Liam Bennett in what he later described as a “perfect punch” after a brief argument during a chance encounter along the canal near the Golden Bridge Industrial Estate.
Mr Bennett, aged 46, suffered a massive brain injury from the impact with the ground and died a week later in Beaumont Hospital.
Garda Kevin Lawless revealed that CCTV footage of the incident showed Mr Bennett swing his elbow at Hatch while the two men were passing each other along the canal.
Hatch, of South Circular Road, Kilmainham, pleaded guilty at Dublin Circuit Criminal Court to the unlawful killing of Mr Bennett at Tyrconnell Rd, Inchicore on August 8, 2013.
He also pleaded guilty to robbing nurse Natalie Delaney of her handbag and contents on Thomas St on January 6, 2013.
He has 29 previous convictions, including a four year sentence for robbery in 2003.
Judge Mary Ellen Ring said Hatch did not set out to kill Mr Bennett but the attack was without cause and gratuitous and nothing Mr Bennett said or did could justify the assault.
“He choose to hit Mr Bennett, causing him to fall on the ground, hit his head and die,” she said.
She suspended the last year of a six-and-a-half year sentence.
Gda Lawless told Garnet Orange, prosecuting, witnesses described Mr Bennett talking to himself as he walked along with an umbrella and rucksack before his encounter with Hatch.
Hatch revealed he had been confused about how Mr Bennett was speaking to himself and had not expected him to “lash out” as they passed each other by.
He said he punched Mr Bennett in response to him saying: “I’ll mill you with this.”
Hatch told gardaí: “I’m not a fighter. I never did boxing or martial arts or anything like that.”
He said he knew it was a “perfect punch”, despite his lack of training.