The Public Accounts Committee is seeking "immediate clarity" in any possible conflict of interest over professional services firm PwC which not only advised health chiefs on the children's hospital but also completed a review of the project.
The committee heard PwC advised last November over the second phase of the children's hospital project.
A review by PwC of cost overruns for the €1.7bn project was only released this week. It was separately commissioned by the government.
PAC chairman Sean Fleming said he wants the Department of Health and the HSE with immediate clarity to say why PwC was chosen for the review after it previously advised about the second stage of the hospital build.
PwC's role in the project emerged in a document discussed by PAC. Three options had been considered as part of the second phase of the hospital build.
And a HSE document confirmed that PwC and others had given advice to the HSE after a request from the Department of Health.
PAC member Marc MacSharry questioned the value of €500,000 PwC's review on the hospital cost and whether it was a “waste”.
Both the HSE and Department of Health have been asked to provide details on why PwC was chosen for the review.
A HSE document called Options Going Forward on the children' hospital detailed how PwC had advised the National Paediatric Development Board and project delivery team about the works.
Options considered included whether construction firm BAM would complete phase B or if a new contractor should be considered.
Mr Fleming said PwC had said there was “no conflict of interest” with the costs review. But people will ask questions, adding:
How is that they were involved in the decision process in relation to how to go forward with phase B [of the hospital] and yet they were commissioned to carry out the costs escalation report?
Deputy Catherine Connolly said HSE document indicated an alternative plan for the hospital build had already been advised against by PwC and others.
Mr MacSharry said PwC had “no place” doing the costs review while Mr Fleming said the situation “affected the credibility” of the people who had commissioned PwC to do the hospital review “if the same people knew [PwC] they were involved in the earlier decision”.
Meanwhile, a former master of the Coombe hospital said it was a "flawed decision" not to have a definitive plan for a maternity hospital on the same site as the new children's facility.
Prof Chris Fitzpatrick said, without a maternity hospital on the St James's Hospital site, up to 200 potentially critically-ill babies yearly would not have access to paediatric facilities.
Children with heart and lung problems would instead have to be driven around in ambulances, the committee heard.
"Critically-ill babies should not be made to suffer because of bad decisions made at the start," he said.
But the Department of Health said the minister has made clear a commitment for a planned maternity hospital on the site.