Improved facilities for prisoners with psychiatric and drug problems would be provided with the increase in prison places, the Minister for Justice, Equality and Law Reform said.
Mr O'Donoghue was replying to the Labour spokesman on justice, Dr Pat Upton, who said the Mountjoy visiting committee's report said 100 prisoners should have been in psychiatric hospitals and only nine were on a drug treatment programme.
Mr O'Donoghue said he had always thought substantial change in rehabilitation and accommodation in prisons would not be possible until there were sufficient prison spaces.
The Government's promised programme was under way, and 1,000 more prison spaces would be provided over the next year.
He agreed that a proportion of prisoners in Mountjoy suffered from a mental illness, but published figures indicated that, at most, 5 per cent had a history of major mental illness. The prison's medical service had advised that not all of them would require hospital care.
Mr O'Donoghue said the drugtreatment facilities in Mountjoy should be extended.