Almost a third of the 33 instances of overcapacity in Mental Health Units last year occurred at University Hospital Galway.
Nationally, the Mental Health Commission's report found there was a major dip in overcapacity on the year previous, when there were 64 instances reported.
2,000 people are living in large mental health institutions, and the report found some residents are sharing rooms, toilets and dining facilities.
The Mental Health Commission is warning the HSE it'll take action and de-register public mental health centres if compliance with regulations is not improved.
Commission Chief Executive John Farrelly says there needs to be a move away from institutionalising mental health patients: