Mountjoy prison bosses have ordered an investigation after €18,000 worth of cigarettes disappeared from its tuck shops in only two months.
An internal audit has found a huge discrepancy in the number of cigarettes being delivered to the jail and the number accounted for through sales in the three shops that prisoners use in the North Circular Road site.
Governor Brian Murphy has ordered an immediate audit to try to get to the bottom of the mystery.
Around €9,000 worth of cigarettes and other tobacco products are unaccounted for in each of the last two months, according to prison sources.
"The first thing in everyone's mind is that inmates are somehow getting their hands on supplies," a source said.
"There are three tuck shops they use - in the Circle, the A-Division and the D-Division.
"The other possibility is that a couple of pallet loads of cigarettes have been misplaced, but there's not that many places in a prison where they could be sitting unaccounted for."
Recently, 170 packets of tobacco were found in one inmate's cell in St Patrick's, which is part of the Mountjoy complex.
But that prisoner insisted he won them playing poker in the jail.
Authorities are now monitoring the situation.
The tuck shops have a monopoly on the selling of cigarettes, newspapers and sweets for the prison population of 520 inmates.
None of them are protected by CCTV, according to prison sources.
Inmates receive a weekly allowance of between €6.65 and €15.40 to spend at the tuck shops, and they are also allowed to receive gifts of money from visitors.
Before 2013, all inmates were entitled to a flat-rate gratuity payment of €2.35 per day.
However, the system was then reformed to provide for three levels of payment to incentivise good behaviour in prisons.
Figures released by the Irish Prison Service in 2013 showed that the tuck shops in Mountjoy had a turnover of €1.3m in 2012, which produced a profit of nearly €125,000.