A hatchet found at the house is being examined as part of the investigation.
A 42 year old man, believed to be a son of the deceased couple, is being held at Buncrana Garda Station under section 4 of the Criminal Justice Act.
Tributes continue to be paid to the slain couple, who have lived in Carndonagh since the 1960s.
Jimmy, a native of County Kilkenny was a retired teacher while Kathleen was a retired nurse.
Local Councillor Albert Doherty knew both of them personally..........