Labour and material costs for the proposed Dublin metro are nearly 2½ times what they would be in Madrid, according to figures provided to an Oireachtas committee.
The figures, compiled by Spanish metro experts, also suggest that a metro contract costing 469 million in Madrid would rise to 1.04 billion in Dublin.
The new figures have led to calls from the Labour Party that the Rail Procurement Agency (RPA) should not be allowed to oversee the construction of any underground rail link.
Ms Róisín Shortall, Labour's transport spokeswoman, also called on the Government to abandon plans for a public-private partnership approach and instead opt for total State control over any metro project.
Ms Shortall was a member of an Oireachtas Transport Committee delegation which was given the figures while on a trip to Madrid last year.
The group travelled to the Spanish capital where 40.5 kilometres of metro was built for just over €2 billion. The project was under total state control.
The group met officials in charge of the project, who said the proposed metro link from Dublin city centre to the airport could be built for just over 1 billion, including land, planning and construction costs, using Spanish methods and contracts.
This compares with 3.4 billion provided by the RPA. Its original estimate was €4.8 billion.
The figures compiled by the Madrid Metrosur office found labour costs to be 238 per cent more expensive in Dublin than Madrid. The main reason for the Dublin labour costs was the high outside consultancy costs. Materials were also found to be 246 per cent dearer, while plant and equipment costs were 143 per cent more expensive.
The Spanish authorities have offered to train Irish State-employed engineers in methods used in Spain to oversee large infrastructure projects. Prof Manuel Melis Maynar, head of the Madrid project, made the offer to the Government in June.
Ms Shortall criticised the failure of the RPA to make public detailed breakdowns of its costs so that they could be compared with those provided in Madrid.
"I think this raises questions about the RPA's ability to deliver the metro on time and on cost," she said. "It's a public body which has been set up specifically to oversee rail construction projects, but quite frankly the experience so far with Luas has not encouraged confidence."
The RPA provided cost breakdowns to the transport committee when it appeared before it in July, but said it was confidential information because of commercial sensitivity.
The committee has criticised this, saying it has meant it has been unable to discuss the source of the cost differences in the RPA's estimates and estimates provided in Madrid.