The Department of Education and Skills claims the cost of repairing structural defects in one of the schools closed following recent inspections is likely to exceed €3.1m.
In Commercial Court papers, the department says although remedial works to the building on the Ardgillan Community College complex in Balbriggan, Co Dublin, have not been fully costed, the €3.1m-plus estimate is based on a quantity surveyor's report.
The department paid the builders, Western Building Systems (WBS), Coalisland, Co Tyrone, €3.5m for the project in 2009.
The department says the building remains closed and alternative accommodation has been arranged for the students.
In an affidavit, Eamonn Murtagh, principal officer in the department, says WBS issued a opinion on compliance on August 21, 2009, confirming the works were in substantial compliance with the building control regulations.
Mr Murtagh says the building does not comply with the regulations or with the department's building requirements.
A fire safety audit was carried out by engineering consultants on behalf of the department and the Dublin Chief Fire Officer also directed that certain fire safety deficiencies be addressed urgently.
The department requested WBS to attend to these works but when it failed to accept responsibility, another contractor, the Coyne Group, was engaged to do the work.
Coyne discovered serious structural issues, apart from fire safety matters, Mr Murtagh says.
These included insufficient wall ties to outer concrete blockwork, inadequate fixings for the sole plate of the stud partition to the floor structure, and inadequate structure framing for large windows.
"Opening up works" confirmed "numerous serious defects", Mr Murtagh says.
After the building was vacated and closed, an exclusion zone was placed around it.
It is considered that external elevations would be vulnerable during storm events.
Engineers have advised it will not be possible to remediate the external wall structure and the entire outer block leaf will have to be taken down and reconstructed.
The department's action against WBS was entered in the fast-track Commercial Court list on consent on Monday.