The Central Mental Hospital broke its approved capacity of female patients last month because of a shortage of space.
It wrote to the health watchdog to say it was at risk of breaching privacy regulations as a result.
Last month a woman was found not guilty of a crime by reason of insanity and the Central Mental Hospital was legally required to find space for her.
At first, the Central Criminal Court heard she could not be committed to a facility because it had reached full capacity in its quota of female patients.
But the following day, October 25, the court was told there was now a place available.
It has now emerged that day Dr Brenda Wright, a clinical director of the National Forensic Mental Health Service in the hospital, wrote to the Mental Health Commission.
In a letter released under the Freedom of Information Act, she said they were legally required to admit a female patient that day who was found not guilty by reason of insanity in the Central Criminal Court earlier in the week.
Dr Wright said this admission meant they would be accommodating 11 female patients, rather than the approved capacity of 10.
She said they had tried to discharge another patient to facilitate the woman but that this was unsuccessful.
The clinical director said they were now at risk of being non-compliant with privacy regulations.
"It is very concerning that the Central Mental Hospital or any hospital would have to go above its capacity to cater for someone in need of acute care," said Brendan Kelly, professor of psychiatry at Trinity College Dublin.
Mr Kelly said that this case highlights the problems at the hospital.
"At any time there are between 25 and 30 people in prison on the waiting list for transfer to the Central Mental Hospital - many of them for many months.
"This is a very difficult situation. Prison is toxic for the mentally ill.
"We need more capacity in the Central Mental Hospital."
The Mental Health Commission says it is satisfied steps were taken to ensure the privacy and dignity of residents in the female unit, after this extra admission.