Kevin McGuirk (53) from Balgriffin, north Dublin, and his wife Kim (34) were found by a business partner at around 10.30am yesterday at their home on Albany Road in the suburb of Alberton in the South African capital.
Mr McGuirk had worked in South Africa as a diving instructor and tour guide for the past 10 years. He has at least two children from a previous relationship, a son who is currently living in Ireland and a daughter who is in South Africa.
The Dubliner's 75-year-old father-in-law, who was living with him and his wife, was arrested at the scene.
South African police said that the elderly man, Erwin Franz Poppinger, had admitted shooting the couple on Monday night.
He was charged with "first degree murder" on two counts yesterday, and if found guilty, he would face a minimum of 20 years in prison.
"When officers arrived at the scene they discovered the body of the man's daughter and her husband lying on the bedroom floor," a police officer said.
"The man had been shot in the back and the woman had been shot in the head.
"Her father informed the officers that he had carried out the killings and his firearms were confiscated.
"He was arrested and is being held in custody."
Mr McGuirk ran a successful scuba diving business with his wife called Africa Diving Tours.
A close friend of the pair, Massyn Jansen Van Vuuren, who gave first aid classes with Kim, spoke of their devotion to each other and their kindness.
"They were both very loving people with a very free spirit," he told the Herald.
"They had a passion for life and for people, it's a real tragedy, they will be sorely missed over here."
He added that Kevin, who had been living in South Africa for over a decade, "loved his job as a driving instructor and tour guide". "He was just so full of life, and constantly looking forward to their next adventure."
Mr Jansen Van Vuuren, who is a member the country's police reserve, was one of the first people informed of their deaths.
"Colleagues of mine who are part of the trauma unit went up to the scene so I learned what happened pretty quickly. We were all very shocked and saddened," he said.
"As far as I understand, there is no doubt that Kim's father did it but none of us understand why."
According to another police source, Kim's father said that he had been mistreated by her and her husband and that they wouldn't let him go out.
The Department of Foreign Affairs said that the Irishman's family in Ireland and in South Africa had been offered assistance.