Gardai are questioning two suspects after a fatal late-night row involving a group of men in a rented house.
Two crime scenes were sealed off by gardai in Tullow and Carlow town, as inquiries into the violent incident were stepped up.
This morning detectives upgraded the investigation to murder.
The dead man has been identified as 41-year-old Latvian, Gints Intembergs, who had been living in the house at Graigowen estate on the outskirts of Tullow.
Neighbours in the estate said they heard shouting and fighting in the house at around 9pm on Monday and heard a car horn two hours later.
But the alarm was not raised until 8.50am yesterday when a friend of the victim, Daiaga Ezermala, contacted the emergency services.
Gardai arrived at the scene and found Mr Intembergs' body lying on the kitchen floor at the rear of his home.
Officers said there did not appear to be much blood on the floor and were awaiting the outcome of a post mortem examination by State Pathologist Marie Cassidy to determine how he died.
A second man, also Latvian, was discovered in a dazed condition with some injuries in the living room.
He was taken by ambulance to St Luke's general hospital in Kilkenny and last night his injuries were described as not life-threatening.
Gardai said the two men appeared to have been violently attacked during the night but initial inquiries indicated there had not been a major struggle or melee in the house.
The body remained in the house until Prof Cassidy arrived yesterday afternoon and carried out a preliminary examination.
The body was then removed to Waterford university hospital for the post mortem last night.
An incident room was set up at Tullow garda station while an investigation team held a full-scale crime conference in Carlow yesterday evening.
A further crime scene was established at a house in Carlow town following the arrest of a suspect, who is Irish.
Officers arrested another Latvian man during a search in Tullow yesterday afternoon.
House to house inquiries were carried out as forensic experts from the Garda technical bureau completed inquiries at the two crime scenes.
Last night senior officers appealed to residents or visitors to Graigowen and Gortnahowen housing estates or Slaney View Drive in Tullow, between 6pm on Monday and 9am yesterday and either heard or saw anything suspicious in those areas to contact them at Tullow, telephone 059 9181160 or the garda confidential line, 1800 666111.
In a separate incident, a man was left critically ill in hospital after an assault in a local authority housing estate in Waterford.
The man was found slumped in an outdoor area in the Priory Lawn area of the estate at around 11pm on Monday.
Gardai are also investigating another incident where a man in his late 20s was discovered in a serious condition after an apparent fall at the Manor Village area of Cork Road in the city.