Staff at Maghaberry Prison near Lisburn have saved the lives of 11 vulnerable prisoners.
Inspectors have expressed concern about the care of such inmates in the past and said plans for some men at risk of suicide and self-harm were unacceptable.
The high-security prison's new governor David Kennedy has said safety was one of his top priorities. Mr Kennedy said officers have rescued 11 lives, 10 last year and one this year.
He added: "The number of prisoners who are kept safe by staff on an ongoing basis through building relationships, it is impossible to measure the good that has been done."
He said staff interventions were crucial.
"If that person who had taken that deliberate case of self-harm had not been inside Maghaberry with their staff, their lives were saved because this is where they are."
A number of tragic cases of self-harm and suicide have been recorded within Maghaberry's walls.
In 2016 a Prisoner Ombudsman report said prison staff watched as a mentally ill inmate blinded himself.
Speaking generally, Mr Kennedy said his staff noticed when behaviour was not normal.
In one case, the governor said, a prisoner would normally have been up and about but was just lying in bed.
The officer checked to find he was self-harming under the covers.
Mr Kennedy added: "There was no reason to check him other than that they had the relationship to observe his behaviour."
The prison has a special regime for those deemed at risk of suicide or self-harm (Supporting Prisoners At Risk) involving intensive monitoring and measures like safe cells with no ligature points.
In 2008 Colin Bell was able to commit suicide. He had been in a safe cell and was deemed at risk of taking his own life and had a history of self-harm.
His death prompted major reforms within the Northern Ireland Prison Service.
Mr Kennedy said often the Service was not aware of the impact of its work in preventing self-harm.
"The number of lives saved through staff relationships and the processes in place, is unquantifiable.
"I think the staff are very proud of what they are doing and people do get a real sense of achievement and satisfaction.
"The people here do make a difference to prisoners' lives.
"Every day they make a difference, sometimes a small difference, sometimes a bigger difference."