MORE than 300 prisoners are allowed out of their cells for just one hour each day, the Irish Prison Service has admitted.
The men are considered "at risk" from other prisoners because of gangland feuds, drug debts or evidence they gave against former associates.
The Irish Prison Service said the actual number of "protection prisoners" in the system was even higher but some could be catered for in a more humane fashion.
However, 308 inmates face a regime of 23-hour lock-up.
Campaigner Fr Peter McVerry said: "If you locked a dog in a room for that length of time, you would be reported to the ISPCA.
"There has to be a different way but the difficulty with doing much about it is the overcrowding.
"There is absolutely no space. Any more than 540 people in Mountjoy is considered not a safe environment and yet last month on one particular evening, there were 791.
"The overcrowding makes it impossible to provide a safe and humane prison environment for people and there is no policy to deal with it. The only policy that exists is to put more and more bunk beds into more and more cells."
Almost half of all these prisoners – a total of 153 men at last count – are being housed in the Victorian conditions of Mountjoy, where they are forced to slop out from their cells.
A substantial number of inmates, 45 at Wheatfield and 40 in the young offenders' institution St Patrick's, are also locked up for all but an hour each day.
Another 23 prisoners in Castlerea are subject to 23-hour lock-up with 26 more – a number of them linked to feuding gangs in Limerick – subject to those restrictions in the Midlands.