Construction work on the scaled-back 'super prison' at Thornton Hall will begin over the next few weeks, following repeated delays and as Mountjoy prison reaches a critical point of overcrowding.

Construction of the access road at the north Dublin site will begin in less than two weeks' time and is expected to take six months. Two separate tenders are soon to be issued for the building of the wall around the prison and the prison itself.

Construction of the prison is expected to begin in January 2011. The prison itself will not be operational until towards the end of 2015 – five years after it was initially planned to open.

Thornton Hall has already run up a bill of €41.8m in site costs, professional fees and security. The 2,200-inmate jail was to have been completed by the Leargas consortium, which included Clare property developer Bernard McNamara, but negotiation between the Irish Prison Service and Leargas broke down last May.

It was initially planned that Thornton Hall be built under a public-private partnership. The state now intends to pay for the prison in annual instalments over 25 years.

In 2005, the government acquired a 150-acre site at Kilsallaghan near Swords, Co Dublin, for €30m to construct the prison.

The price per acre was far higher than any other in the area at that time, prompting criticism of then justice minister Michael McDowell.

A report by the Inspector of Prisons published in September 2009 found there was "inhuman and degrading treatment" at Mountjoy Prison.

It also found it was the most consistently overcrowded prison in Ireland.