There are only two beds in the Central Mental Hospital for every 100,000 people in the country, according to Fianna Fáil.
The party is raising serious concerns about capacity issues in the south Dublin facility.
It comes after a woman was found not guilty of the murder of her child this week by reason of insanity, but was turned down for a bed in the hospital because it has no room, and is now out on bail.
Fianna Fáil's mental health spokesman, James Browne, said it is "inhumane", and is calling on the State to act.
Mr Browne said: "Well I think it needs to be a stronger issue for the Government, they need to prioritise mental health care, I think that simply hasn't been there.
"In particular, our prison services need mental health services, the funding hasn't been put in there and a plan has not been put in there either.
"There are only two beds to every 100,000 people in this country, there needs to be significantly more than that and the Government needs to be planning now to provide the services that are needed."