Four inmates at Mountjoy Prison attacked a young male prison officer last April because they wanted to get their hands on the bunch of keys the officer was carrying.

The keys were to open a gate separating them from another section of the Dublin prison’s Dwing. It later emerged that more than a dozen of their associates were waiting, ready to stage a riot if the barrier could be opened using the officer’s keys.

The young prison officer single-handedly fought the inmates and managed to stop them getting the keys before assistance arrived from a number of colleagues who, fortunately, were carrying out a riot drill at the time of the attack. But prison officers at Mountjoy say they can not indefinitely rely on overstretched prison staff being able to resist such attacks.

The 158-year old jail is creaking at the seams. It has a capacity for 540 inmates but the numbers have been consistently higher for months - with more than 630 inmates sleeping everywhere from the reception area to concrete floors and in cleaning cubicles.

As the state’s main remand centre, Mountjoy is suffering more than the other 14 jails under the control of the Irish Prison Service (IPS) from a dramatic surge in the country’s prisoner population over the past 12 months.

Last October, the numbers in Irish jails swelled above 4,000 for the first time in the history of the state. Since then, they have grown a further 10 per cent. The increase is partly attributable to a decision by the Garda Commissioner Fachtna Murphy last year to proactively enforce outstanding warrants for arrest and committal.

The success of Murphy’s crackdown has been made possible by the growth in the numbers of attested gardaí - up from 12,000 in 2004 to a predicted level just below 15,000 later this year.

But the subsequent overcrowding crisis has led to a dramatic upswing in the number of criminals being released on temporary release orders (TRs) at the discretion of prison governors. In October 2008, more than 360 offenders were on temporary release out of a population of 4,012 prisoners. This compared to 189 on the same date in 2007 out of a prison population slightly below 3,500.

Last month, more than 560 offenders were out on TR orders out of a prison population of 4,430. But even though almost 15 per cent of prisoners are out on TR, the occupancy rate is still running at 106 per cent across the state’s 15 prisons.

The problems posed by violent inmates cramped into Dickensian conditions amid widespread drug-taking and substance abuse are arguably at their worst in Mountjoy.

The north Dublin city jail is the main place of remand for males over 18.The government wants to knock it down and replace it with a new facility, but this will not open until 2013 at the earliest.

Prison officers believe people on the outside do not realise how dangerous conditions are - for both staff and inmates - nor how decrepit the living conditions are for prisoners.

Over the past fortnight, two separate incidents resulted in two male prisoners and a female prison officer being admitted to hospital.

One of the prisoners, who was beaten over the head by a bag containing pool balls in a row over a television remote control, remains in a serious condition while the other two are recovering in hospital.

Mountjoy chaplain Father Charles Hoey last week described the prison as a ‘‘volcano waiting to erupt’’ following a spate of violent incidents.

The cleric’s comments were branded as ‘‘irresponsible’’ by Brian Purcell, director general of the Irish Prison Service (IPS), who warned that they could create a self-fulfilling prophecy.

One senior Mountjoy prison officer last week told The Sunday Business Post that most officers at the prison felt Hoey’s comments ‘‘showed he was more in touch with the reality of what was happening in the prison than the head of the IPS’’.

When the last major large scale incident occurred at Mountjoy prison in August 2008, officers were ill equipped to tackle 70 rioting inmates. They were only able to get their hands on a small amount of anti-riot gear as the rest - including shields and helmets - were stored in a room that was not accessible at short notice.

Torches and fire extinguishers, needed to put out small fires started by the rioters, did not work. Some prison officers had to wait for torches to be sent from St Patrick’s Institution before they could enter the blackened riot scene as batteries in their own torches had run out.

Prison staff later learned from inmates that the rioters had planned a full-scale takeover of the jail, but it did not happen because of the mistiming of a similar attack in a separate wing, which was supposed to coincide with the first riot. The IPS said it was attempting to deal with the current overcrowding issue but it ‘‘cannot decline to accept people assigned to it by the courts’’.

Until recently, a spill over of prisoners at the facility slept in Mountjoy jail’s enclosed reception area before governor John Lonergan ordered the practice to end for security reasons. When the state’s statutory inspector of prisons, Judge Michael Reilly, visited the prison unannounced on November 22 last year, he came across inmates bedding down on dirty and soiled mattresses.

In some cases, the prisoners were without any mattresses to lie on. Many of the mattresses that were there were burnt with cigarette burns or stained from the unmistakable patterns of a previous user having been sick on the bedding. In his first interim report last October, Reilly said that overcrowding, violence, drug abuse and the lack of treatment for mentally ill inmates were issues of major concern in the Irish prison system.

It is believed Reilly has also been briefed by prison officers about the unavailability of riot gear for inclusion in his final report.

Drug-taking continues to be a major problem in prisons, fuelling violence and creating a competitive market for illicit produce behind bars.

The state’s programme of drug testing in 2008 was its most comprehensive annual assessment yet - 27,209 samples were forwarded for analysis to check levels of opiate, benzodiazepine, cocaine, cannabis and other substances.

Some figures show a small decline in the number of prisoners taking drugs. But more than half of the prisoner population in Mountjoy - 52 per cent - tested positive for opiates.

The figures are based on a full breakdown of the substance use analysis carried out by the IPS over the 12 months and a single test also carried out in January 2009, which were obtained by The Sunday Business Post under the Freedom of Information Act.

In the main Mountjoy prison, 57 per cent of all tests carried out in the first nine months of 2007 showed a hit for opiates. Tests conducted over the 12 months of 2008 recorded 52 per cent hits. That fell to 50.9 per cent in testing of 418 samples in January of 2009 - but this sample was only 13 per cent of the size of the data produced in 2008 and may be adjusted due to seasonal patterns.

Benzodiazepine - a psychoactive drug which can include horse tranquillisers and anxiety medication - continues to be a major drug of choice among the Irish prison population.

Tests showed 48.9 per cent of inmates at Mountjoy main prison were positive for benzodiazepine and 50.7 per cent tested positive for cannabis in January 2009, compared to 49.3 per cent and 45.4 per cent respectively in 3,279 tests conducted in 2008.

In the first nine months of 2007, 2,284 tests on inmates at Mountjoy women’s prison revealed 53 per cent positive hits for benzodiazepines compared to 57.7 per cent over 2008 and 46.9 per cent in January.

Seven per cent tested positive for cocaine at the women’s jail compared to 1.6 per cent at the men’s jail in January. Cocaine tested low or at almost nil across most jails except Cloverhill prison, where it was at 7 per cent, and Arbour Hill, where it was at 11 per cent. Both Cloverhill and Mountjoy are remand prisons and they are likely to show a disproportionate level of positive tests which relate to drugs taken prior to committal.

In 2008, there were 3,191 drug tests carried out at Cloverhill of which 36.9 per cent were positive for opiates and 39.4 per cent were positive for benzodiazepines, while 26.7 per cent recorded hits for cannabis.

Two years ago, 37 per cent of samples tested at Limerick jail showed positive for cannabis compared to 31 per cent in 2008 and 24 per cent in January 2009. In 2007, 45 per cent were positive for opiates, compared to 37.7 per cent in 2008. Benzodiazepine use remains steady.

In Wheatfield Prison in Dublin, 49 per cent of inmates tested in 2007 were positive for cannabis compared to 40.2 per cent in 2008.

St Patrick’s Institution for Young Offenders in Dublin continues to show some of the lowest rates of drug taking in the system, with only 5.4 per cent positive for cannabis in 2008, compared to 7 per cent in 2007.