Damien Duffy is understood to have killed his father Daniel at their bungalow at Windyhall on the outskirts of Letterkenny, before taking his own life.
Daniel (88) was found inside the house while it is believed his son, aged 50, was found in a car at the rear of the property.
The deaths have sent shockwaves through the local community where the family were well known and highly respected.
Gardaí are understood to be treating the deaths as a suspected murder-suicide and are not seeking anyone else in relation to the tragedy.
Both men were described as “extremely intelligent and engaging” by neighbours.
Mr Duffy Snr was head of the ESB in Letterkenny for a number of years.
Damien also worked with the ESB for a number of years before leaving.
Damien was regarded as a computer genius and often worked in a self-contained flat at the back of the pebble-dashed bungalow.
It is understood a neighbour who called to the house around 2pm on Thursday made the gruesome discovery.
Gardaí immediately sealed off the area and the scene was preserved awaiting the arrival of the State Pathologist and the Garda Technical Bureau.
It is believed Mr Duffy Snr had not been seen in around a week and Gardaí are now trying to determine when he actually died.
Neighbours of the men said they were both “witty and engaging” and that Daniel simply “idolised” his son Damien.
Damien recently graduated with first class honours in computing and had planned to do masters next year.
Both were huge Manchester United fans and were both season ticket holders often going to matches at Old Trafford.
They also loved snooker and were very creative, often working on building projects together.
The mother of the family, Christine, passed away in 2009 and the two men lived in the house together.
The family suffered another tragedy two years ago when Mr Duffy’s daughter Caroline died from cancer aged just 48. Another daughter lives nearby.
A neighbour and lifelong friend said: “Daniel and Damien were two of the finest people – hard working, highly intelligent, creative and very entertaining. This is incomprehensible. I am devastated and will miss them terribly.”
This afternoon, locals passing the family home blessed themselves as they slowed down as a mark of respect to the tragic family.
A number stopped in a bid to catch the family’s pet dog which ran down the road from the scene of the tragedy.
A Christmas wreath hung from the front door of the bungalow while a Christmas tree stood in the garage window at the rear of the house.
Two large satellite dishes were erected onto the garage which were believed to be part of Mr Duffy Junior’s work with technology.
Local councillor Gerry McMonagle said the whole community is stunned by the news.
“It’s a dreadful tragedy our hearts go out to the families involved,” he said.
“The whole community is stunned, it’s a nice quiet area, people just can’t believe it.
“People feel they have troubles but tragedies like this put life in perspective.
“Our thoughts and prayers are with the family all connected with this.”
He added: “It’s an awful tragic incident all the more so when it’s father and son and we are just two days away from Christmas.
“Our thoughts and prayers are with all the extended family at this time.”
Councillor Michael McBride echoed the shock of the local community.
“It’s a real tragedy especially at this time of the year. Our hearts go out to the family and the neighbours there, it is an area we would never associate with any kind of violence.
“There is deep shock in the community tonight.”