Thomas and Paddy O' Driscoll, who were nine, were found at their home in Charleville with stab wounds whilst their older brother, 22 year old Jonathan, was found 15kms away.
A post mortem examination is due to be carried out.
The police in Ireland have said it's not looking for anyone else in relation to any of the deaths.
The Bishop of Cloyne, Rt Revd William Crean, sent his 'deepest sympathy' to the parents, Thomas and Helen O'Driscoll.
He said: "I speak for our faith community, and for all people of good will, when I say how shocked and greatly saddened I was to hear of the tragedy that has befallen the O'Driscoll family.
"Such loss of human life is extremely difficult to comprehend.
"I pray that the Lord will strengthen the family during this terrible time.
"I ask the priests and people of the diocese to pray for Thomas and Helen, for their surviving children, their extended family, and for the community of Charleville."