The body of Mr Boyce, 72, was discovered in an upstairs room of his home at St Munchin’s Street, St Mary’s Park, after neighbours raised the alarm when he was not seen for a number of days.
He was beaten to death and his body locked into the room.
Sources yesterday indicated that the investigation has now identified a man aged around 20 who is the main suspect.
He knows the St Mary’s Park area very well, and lives in another part of the city.
It is believed the suspect was recently released from prison and is known to be a very violent individual.
He may have attacked Mr Boyce when he went to his house to steal money to buy drugs.
Garda forensic experts completed their examination of the house yesterday.
Mr Boyce served in the army from 1956 to 1962 and spent six months with an Irish UN contingent in the Congo in 1961, when his unit was attacked by the Baluba tribe.
He was awarded a UN medal for his service in the Congo.
On leaving the army he emigrated to England.
He returned to the family home in St Mary’s Park 16 years ago on the death of his mother, Mary Ellen. As his brother, Joseph, was ill at the time, he remained to look after him and never returned to England.
Mr Boyce was living alone at the old family home and was described by shocked neighbours as a very quiet man and one “of the old stock” of the parish.
Supt Frank O’Brien, who is leading the investigation, said they had got a huge response from the local community.
Detectives have been trawling through hours of film harvested from the street CCTV system in St Mary’s Park.
There are believed to have been various sightings of the main suspect in the St Munchin’s Street area between Saturday morning and Monday last.
Mr Boyce’s funeral will take place on Saturday after Mass at St Mary’s Church.
His brother David and sister Noreen are due in Limerick today from England for the funeral.