Private independent mental health services typically offer a better level of care than HSE-run facilities.
The Mental Health Commission says a "select number of people" with health insurance or financial means are able to access the highest-performing centres.
The watchdog is calling for significant State investment to make sure everyone has access to a similar standard of mental health care, regardless of their income.
Mental Health Commission CEO John Farrelly says the gap between private and public care is far too wide.
"To become an in-patient, means that you are at the lowest you will ever be in your life.
"It's not fair that the independent and private centres have these high standards in their buildings and in patient centred services, and a lot of the HSE services don't.
"It's quite simple that the HSE and how it manages its mental health service really needs to focus on the jobs to ensure that all citizens in Ireland receive appropriate care."
