The three family members who were found dead at their home in the Drumgowna area of Co Louth have been named locally.
They are Mark and Louise O'Connor, who were aged in their 50s, and their son Evan, who was aged in his 20s and had additional needs.
Evan was attending a day care centre in Drogheda, it is believed when staff called to the house to collect him this morning, the bodies were discovered.
Gardaí are investigating what they believe is the murder of the three family members.
They were called to a house in the Drumgowna area this morning.
The house is located close to the Louth/Monaghan border near the villages of Killanny, Louth village and Tallanstown.
Several garda units, including the armed response unit, raced to the scene after the alarm was raised at around 10 o'clock this morning.
It is believed that all three people were violently attacked and sustained fatal injuries.
A man in his 30s has been arrested and is being held at a garda station in the northwest region. He is being detained under Section 4 of the Criminal Justice Act and can be held for up to 24 hours.
It is understood medical assistance is being sought for him.
The area has been sealed off and the State Pathologist's Office and the Garda Technical Bureau have been notified.
The bodies of the three people were taken away in two hearses a short time ago to Our Lady of Lourdes Hospital in Drogheda. Post-mortem examinations are expected to be carried out tomorrow.
Gardaí are not looking for anyone else in relation to the incident at present and there is no threat to the public.
Watch: North East Correspondent Marc O'Driscoll reports from scene in Tallanstown
A large garda presence remains at the scene this evening and all approaches to the house, which is a stone cottage, have been sealed off.
Members of the Garda Technical Bureau have attended the scene and a blue tent was erected in the driveway of the house this afternoon.
Meticulous searches were carried out both inside and around the outside of the property as forensic personnel look for anything of evidential value.
It is understood that this investigation is being led out of Drogheda Garda Station, where gardaí are holding an internal conference this evening to discuss how next to proceed with their investigations.
The investigation is being conducted from an incident room at Ardee Garda Station and a Senior Investigating Officer has been appointed.
A Family Liaison Officer has been appointed to support and keep the family of the deceased informed.
Local representatives have also asked people not to share false information about the incident that has been circulating online.
Minister for Justice Jim O'Callaghan said gardaí are not looking for anyone else in respect of "this criminal act" and there is no ongoing threat to the public in Co Louth.
"There's no further information I can put into the public domain at present, but I think further information will become available in due course," Mr O'Callaghan said.
Watch: Minister for Justice Jim O'Callaghan says gardaí not looking for anyone else
Family very well known in area, says councillor
A local councillor said the family is very well known in the agricultural area between the three villages of Tallanstown, Louth village and Stonetown.
Speaking on RTÉ's News at One programme, Councillor John Sheridan said he would have been at the house during local elections. He said he would have known the family through local community and sporting events over the years and they are very well known in the area.
"We hear of tragic events quite often. We don't expect them to happen in our own locality. There is a huge degree of shock in the area," he said.
"There are garda cordons in place so it's not possible to get near the house. There were a lot of reports this morning of emergency vehicles coming from Dundalk and obviously gardaí are now following up in relation to the incident."
Local TD expresses shock at incident in 'quiet, rural area'
Sinn Féin TD for Louth, Ruairí Ó Murchú, expressed his shock at the news of the deaths of the three people.
"This is a truly shocking and appalling incident which has absolutely stunned the whole community all over County Louth," he said.
"The fact that three members of one family seem to have died in dreadful and violent circumstances on a Monday morning in a quiet and rural area of Mid Louth is appalling.
"I would appeal for anyone with information to go to the gardaí immediately," said Mr Ó Murchú.
Additional reporting: Dyane Connor
