The figures, released by the Irish Prison Service (IPS), indicate that the inmates either identified that they had a drug problem while in prison or became addicted to heroin while serving sentences.

On 31 December last, 472 prisoners presented themselves to the medical wings of Irish jails seeking methadone in an effort to beat their drug addictions.

Some 162 of those inmates had never received the heroin substitute before and were given the syrup for the first time.

When they were committed to jail, they did not indicate that they had a drug problem and had never sought treatment for heroin dependency.

The IPS says the figure highlights the success of its new strategy aimed at curbing the use of drugs in jails, but the figures could also indicate that some of the 162 inmates tried heroin for the first time while in prison.

A total of 1,363 individual prisoners were given methadone across the prison network last year. Some 12,157 people were sent to jail last year, meaning that 9% of the total population had a heroin addiction.

The total figure is likely to be far higher however, because it does not include prisoners who sought counselling or treatment for drug addictions other than heroin. An average of around 3,500 people are in jail in this country at any one time.

Mandatory drug testing for all prisoners is set to be introduced in the near future as part of the new IPS strategy for reducing the chronic availability and use of drugs in jails.

The IPS says significant improvements have been made as a result of its 'Keeping Drugs Out of Prison' strategy. A total of 24 addiction counsellors were hired last year and all prisons now have dedicated drug treatment teams made up of officers and nurses. Treatment includes detox programmes and vaccinations.

John Lonergan, the governor of Mountjoy jail, recently said that the problem of drugs brought into the jail was as bad as ever.

He said that as well as prisoners being addicted to heroin, there were a large number of inmates also using cannabis, sleeping pills, valium and prescription drugs.

An IPS spokesman said: "Due to a wide range of providers, including prison healthcare staff and community and voluntary bodies, that currently provide drug treatment services, there is not an easily accessible centralised database of those availing of treatment services."