The revelation came as gardaí began interviews with heartbroken Anne O'Sullivan (60), who saw her eldest son Mark (25) ambushed and shot by his father Tadg (59) and younger brother Diarmuid (23) at the family farm at Assolas outside Kanturk in north Cork on October 26.
Ms O'Sullivan, who has been battling serious health problems in recent months, is being interviewed by gardaí.
It is now believed Mr O'Sullivan, who worked in a Charleville garage, was the main influence in the dispute.
His younger son Diarmuid, who was to get a small field of poor land under the inheritance, felt he had been excluded in favour of his older brother.
It is also thought Diarmuid followed the lead taken by his father in the tragedy.
Mark, a trainee solicitor, was shot and killed following a confrontation with his father and brother in the bedroom.
The confrontation was sparked by a row over the inheritance of 115 acres.
A family feud over the farm escalated when details of a proposed will became known just over two weeks before the shootings.
After Mark's shooting, Mr O'Sullivan and Diarmuid went to a field some 600 metres from the Assolas farmhouse and took their own lives.
It has emerged a man contacted gardaí about conversations he had with Mr O'Sullivan on three occasions at the garage where the father of two worked.
In each case, Mr O'Sullivan asked about the legacy impact of such tragedies on surviving family members - specifically referencing one previous Irish murder-suicide.
Legal correspondence showed a solicitor's letter had been issued by Mr O'Sullivan in relation to the land dispute.
Detectives now believe Anne and Mark O'Sullivan were deliberately lured back to the farmhouse on October 25.
Mark had taken his mother, a nurse who worked in Mallow, to Dublin for a medical appointment the previous week.
When they travelled back to north Cork, they did not immediately return home.
They did return on the Sunday night before the triple tragedy, apparently having been led to believe that an amicable solution to the land dispute could be found.
Gardaí fear Mr O'Sullivan deliberately brought them back to attack his eldest son.
A key element in this plan was a letter he sent his wife from a Cork solicitor in which he held out the hope of reaching a settlement over the will.
His wife got the letter and decided to return home with her elder son.
Ms O'Sullivan heard the sound of shots from her eldest son's bedroom just after 6am on the Monday and was horrified to find him fatally wounded - her husband and younger son left the farmhouse armed with rifles.
Gardaí believe she was deliberately left unharmed in order for her to suffer the pain of having lost her beloved son for the rest of her life.
The last words spoken to her before she fled to raise the alarm was a reference to the will and the inheritance.
Mr O'Sullivan and Diarmuid left a detailed note, found by Diarmuid's body, which was marked for Anne's attention.
This note blamed the tragedy on the will - and claimed that advice believed to have been given by locals about not splitting the farm was also a feature.
Bizarrely, it also instructed Ms O'Sullivan to care for the family pets.
Letters from Mr O'Sullivan have also been studied by detectives.
A letter written by Mark underlined his fears that the dispute would split the family.
He also voiced his private concerns at how upset his father and younger brother were - and what potential consequences might arise.
Counselling support services are being made available for people in the community traumatised by the triple tragedy.
Gardaí are awaiting the results of ballistic and forensic tests.
Mark was shot up to seven times.
The young law graduate had injuries to his hands - indicating he had fought for his life.
After Mark's shooting, Mr O'Sullivan and Diarmuid went to a field called 'The Fort' where they are suspected to have taken their own lives.
It is believed the note found by Diarmuid's body was written several hours before the shooting - showing the attack was premeditated.
Elements of the note, which runs to a dozen pages, were described as "very upsetting" by one source and indicated Diarmuid may have been influenced by his father.
A file will be prepared for the North Cork Coroner with an inquest to be staged next year.