Communications Minister Richard Bruton has reiterated his rejection of advice from the head of the Department of Public Expenditure and Reform, Robert Watt.
Mr Watt controversially told the Government to abandon the National Broadband Plan.
Mr Bruton said it was the job of Mr Watt's Department to question Government decisions.
"The Department of Public Expenditure and Reform are doing their job and it is their job to be sceptical about every investment that is ever considered," Mr Bruton said.
He made the statement as the Government has faced sharp criticism from opposition because the private company involved is only investing €220m up front although the entire project will cost €5bn.
Minister Bruton was responding to concerns from Senators during statements on the National Broadband Plan in the Seanad.
He said the advice from the Department of Public Expenditure and Reform "was not the final word."
He added: We have made a decision.. that this was something worth doing."
The Minister said every other department submitted views in favour of the project.
He acknowledged to Independent Senator Gerard Craughwell that it was unusual to reject the advice.
He also said the government would "ride shotgun" to ensure the €480m contingency fund is not drawn on unless absolutely justified.
Mr Bruton also said there was no reason to believe that the cost structure delivered by the EBS would be cheaper than that offered by Eir, or Granahan McCourt, who he said made comparable bids on costs.
He also said that the state is "no way exposed to any escalation in cost" in terms of the take up of the National Broadband Plan.