The PWC investigation says re-tendering the project is not a feasible option and would likely increase costs.
The PWC report warns the 1.7 billion euro pricetag for the new National Children's Hospital could rise even further.
It says if construction inflation increases to 10 per cent it could add 97 million euro to the pricetag.
Some of the design hasn't been finished and some parts haven't been costed - which could see the project cost even more.
There's already been a 450 million euro increase in the cost of the hospital - and this report says just 4 per cent of that was uncontrollable.
The bulk of the overrun was down to an underestimation of costs.
The report says by the time the cost overruns became clear it was too late to re-tender the project - and doing so would have increased costs, delayed the completion of the hospital and risked it not being built at all.
The report says there's very few areas where money can be saved or recovered.
Taoiseach Leo Varadkar has described it as grim reading, and says the priority is getting the hospital built and learning from mistakes made.
Wicklow TD Simon Harris says he has asked the chair of the board to look at the potential for cost reduction.