Fr Paddy O'Kane, 73, was found dead in the garden of a hospital at Gransha Park in Derry on 28 March 2022.
He was being treated for depression at the hospital at the time of his death.
At an inquest into his death on Wednesday, an assistant director of the Western Health and Social Care Trust said the garden area had not been included in a health and safety assessment.
This “was a serious and grave error”, the trust’s assistant director for older people John McGinley said.
Mr McGinley told the court in Belfast there had been lessons learned from the tragedy and that he was “very sorry for our failings.”
He said it was realised afterwards that “the garden was unsafe”.
An action plan set in motion after this incident, Mr McGinley said, means no one is now in the garden unsupervised.
'Clearly helpful'
A number of other safety issues have also now been addressed, he added.
Earlier, Dr Conor Barton had told the inquest Fr O’Kane was being treated for depression and a persistent depressive condition.
A decision had to be made as to what level of observation there should be on a patient, Dr Barton said, adding that that decision was between general or continuous observation.
The witness said “the garden was clearly helpful” to Fr O'Kane whose mood was said to be low in the mornings.
The hearing continues.