A murder investigation was under way last night after an elderly patient in a hospital psychiatric unit was smothered to death with a pillow.
Gardai detained another patient for questioning and he was in custody last night.
The death of the 82-year-old man, named locally as Micheal Traynor of Carrickroe, Co Monaghan, was discovered by staff at the mental health acute inpatient unit, attached to Cavan general hospital, around 7am yesterday.
Gardai were immediately notified and arrested a suspect, in his 40s.
He was being questioned last night at Cavan garda station.
The deceased man's body was removed to Louth county hospital in Dundalk for a post-mortem examination by State Pathologist, Dr Marie Cassidy, who was expected to confirm the cause of death.
Officers waited for the post-mortem result before officially declaring that their investigation was a murder inquiry.
Staff at the hospital said they were shocked by the incident.
In a statement, the Health Service Executive said it was examining all of the circumstances surrounding what it described as "an unexpected" death.
The HSE said it was co-operating fully with garda inquiries and providing support to patients, their families and hospital staff.
It confirmed that it was following appropriate procedures and had informed the Mental Health Commission, as required by legislation.
It pointed out that a review of the circumstances was normal procedure for any unexpected death of a service user and said no further comment could be made at the moment, for reasons of confidentiality and with its own review and the garda investigations under way.
Locals in the tiny village of Carrickroe were shocked and saddened last night at the news of the death of their neighbour and friend.
"He was known locally as Mickey Crush, that is how people would have known him," said one elderly woman who did not wish to be named.
"He was a lovely, gentle man and a great farmer. He lived with his brother until he died and when his sister died some years back he lived alone. Sadly he developed Alzheimers and had to be admitted to hospital about two years ago. We are all so sad today about what happened. It is a terrible tragedy."
Mr Traynor's wake will be held in the family home in Dava, Carrickroe, Co Monaghan.