Seven inmates released temporarily over Christmas have failed to return to prison and are now "unlawfully at large" and being sought by gardaí.

Some 176 inmates were released for short periods over Christmas. This was a substantially higher figure than the year before when 107 prisoners were offered short periods of temporary release for the festive period.

The reason the number has increased is because the prison population has risen significantly. None of the seven inmates who have so far failed to come back are well-known criminals.

"Seven prisoners are unlawfully at large. We have alerted local gardaí. We expect they'll either return themselves or be returned by gardaí," said a spokesman from the Irish Prison Service.

If some of the inmates fail to return within the next week, the prison service may release details of those unlawfully at large. When and if they return, the inmates will face possible sanctions by the prison authorities, depending on their circumstances and the explanations they provide for failing to return on time. The length of their remaining prison sentences will remain on hold until they return.

The prison service spokes­man added that those selected for temporary release over Christmas were chosen "with public safety being the primary concern".

The 176 inmates released were freed for varying periods. Convicted murderer Malcolm MacArthur was released from Shelton Abbey prison for a short period on Christmas Day so he could spend time with family members in Dun Laoghaire. He returned to prison at the agreed time. It was his third period of temporary release.

MacArthur (61), received a life sentence in 1982 for bludgeoning a nurse, Bridie Gargan, to death in the Phoenix Park, Dublin, that year. He was also charged with shooting a farmer, Donal Dunne, who had met him to sell a shotgun, but was not prosecuted for that killing.

The temporary release of certain inmates for Christmas has been practice for decades.

The process is provided for under the Criminal Justice Act 1960. Minister for justice Dermot Ahern said the numbers released this Christmas represented 4% of the prison population.

The year before, 107 inmates were released, compared to 138 in 2007 and 238 in 2006.

Ahern said the prisoners being released were nearing the ends of their sentences. In previous years, a small number of inmates released for short periods over Christmas failed to return to jail on time.