Gardaí believe Mark O'Sullivan was shot in his bedroom by his father and younger brother in their home in Kanturk on Monday morning.
The indications are that 59-year-old Tadgh and 22-year-old Diarmuid shot themselves a short time later in a nearby field, near a fort.
Gardaí have also found correspondence from one of the men which is believed to be crucial to gardaí in helping them establish the happenings at the farm at Assolas from 6.30am on Monday morning.
Post mortem examinations on both Tadgh and Diarmuid were conducted yesterday by Assistant State Pathologist Dr Margaret Bolster.
She will begin a post mortem on the body of 25-year-old Mark this morning at Cork University Hospital at 11am.
A garda source said this morning: ''The indications are that the Tadgh and Diarmuid killed themselves.''
The bodies can be held for up to five days under the Coroners Act.
The deaths came just days before Diarmuid was due to graduate from Cork Institute of Technology with a first class honours degree in accounting. Mark was a graduate of UCC.
Mr O'Sullivan's wife Ann managed to flee the house at 6.30am and raised the alarm when she arrived at a neighbour's house about a gun incident in her home.
Two firearms were found in the field and have been taken away for analysis.
The incident is understood to be linked to an inheritance dispute which had been simmering in recent weeks between the two brothers.