The remains of Jack Blaine, 70, and his brother, Tom, 68, were discovered by a care assistant who called to their house at New Antrim Street near Castlebar town centre at 7am yesterday.
The results of an autopsy were expected to show the two men had been badly beaten.
One body was found in a bedroom with “obvious visible injuries’ according to gardaí, while the other body discovered in the back yard, had “visible injuries”.
It is understood a large amount of blood was found at the scene.
Garda Supt David Taylor would not comment as to whether the victims had both been beaten to death.
“It’s too early to say yet… the results of postmortem examinations and the Garda technical examination of the scene has yet to be completed,” Supt Taylor said.
The man arrested was last night being held at Castlebar Garda station under section 4 of the Criminal Justice Act 1984.
State pathologist Dr Marie Cassidy carried out a preliminary postmortem of the bodies at the house yesterday.
Later, both were taken by Kilcoyne Funeral Undertakers to Mayo General Hospital for a full postmortem.
While remaining tightlipped yesterday evening about their investigations so far, gardaí appealed to anybody who may have spoken to the men late on Tuesday night or early yesterday to come forward.
A Garda source described the two men as vulnerable and said they were last seen at around 10pm on Tuesday sitting in front of their house, where they would often be seen greeting locals and passers-by.
Both victims, especially Jack, were well known in the Castlebar area and would briefly frequent public houses in the area at night where he would get complementary mugs of tea or coffee.
Local parish priest Fr John Cosgrove summed up the feelings of the local community when he described the Blaines as “simple, gentle, happy, fellows. The violence involved is worrying, especially for elderly people”.
New Antrim Street is a busy part of Castlebar town centre, with pubs and restaurants nearby.
It is understood the street outside the house is covered by CCTV from nearby pubs.
The town was reportedly quiet last night.
An incident room has been set up at Castlebar Garda Station which can be contacted on 094 9022222.
Taoiseach Enda Kenny has described the brutal murder in his home town of two elderly brothers whom he knew very well as an act of savagery.
The pair, who were not married and had lived together all their lives, were discovered in their terraced home in New Antrim Street, Castlebar, Co Mayo, early yesterday morning.
Gardaí believe they died after being violently attacked.
A large amount of blood was at the scene where the bodies were found by a home help.
Mr Kenny, who has his constituency office on the same street, said: “I was shocked this morning to get a call from my own town where there has been the brutal and savage murder of two men I knew well.”
He described the murders as an act of savagery.
“I have spoken to the gardaí and I know they are pursuing lines of inquiry, and I hope that person or persons responsible are brought to justice in due course.”
Mr Kenny described the elderly brothers as “lovely people”.
“They never harmed anybody in their lives, were residents of the street where my office is, I knew them very well,” he said.
“I’m very sorry to hear of these brutal murders.”