It is alleged Breen was abusive and would threaten violence, claiming they had thrown rubbish into his yard.
On the morning of his death, Mr Monahan had bumped into Breen in a nearby shop after a drinking session.
When he returned home, Breen was standing outside "clearly annoyed".
Mr Benson said when Mr Monahan went over to him, he was punched to the left side of the neck, causing him to drop to the ground.
Mr Benson said this brought Breen to his senses and he immediately apologised and tried to help him.
The trial continues.
He added: "Unquestionably in this case, the defendant regrets causing the death of Mr Monahan."
