John was a native of Kinnegad and a father of three grown-up children.
His sister Mary Traynor, who lives nearby, told the Herald today that he had lived a troubled life in recent years.
"To be honest, I'm surprised he lived as long as he did. I always had a feeling that his life would end in some sort of tragedy," she said.
"There have been times when ambulances had to be called there before. It's very sad but he was a heavy drinker."
Mary was speaking after it was confirmed that John and Debbie had died of hypothermia.
Neither of them had eaten in days, and both also had health problems.
Deputy State Pathologist Dr Khalid Jabbar said that in John Glennon's case, a serious heart problem had been a contributory factor; while Debbie McEvoy had suffered from high blood pressure.
The two friends were found lying together on the floor of Mr Glennon's flat in a sheltered housing unit at the Drumalee Court complex in Stoneybatter.
The grim discovery was made by gardai shortly before lunchtime on Monday after officers were alerted by health board officials, who became concerned when they were unable to make contact with the two.
It's believed that they died from the cold over the weekend.
While it was first thought the pair may have died from carbon monoxide poisoning, it has been revealed that there was no heat on in the house when the bodies were found.
Bord Gais said: "Gas supply to the property ceased in 2007 at the request of the customer and has not resumed since then."
John had an electric heater but it was not plugged in when their bodies were discovered.
Mary said her brother, another sister, and her parents had moved from Kinnegad to Clonsilla when they were teens.
She said: "John set up repairing televisions, but ended up getting shot in the leg in some sort of row.
"He was drinking heavily and his life turned into a disaster," Mary added.