Work to preserve ancient monuments in the path of a new motorway running through Co Meath could cost up to €20m.
Archaeological work is being carried out on the 32-mile stretch of road between Clonee and Kells.
The National Roads Authority has already spent €10m protecting ruins along the M-50. About €6m of that figure was spent at Carrickmines Castle.
The escalating cost of building motorways means taxpayers may face more tolls than expected.