The Health Minister says it's an understatement to say he was very disappointed and frustrated with the spiralling costs of the National Children’s Hospital.
Simon Harris is appearing before the Health Committee to discuss the project.
The estimated cost has risen from an original estimate of €650 million to the latest estimate of between €1.4 billion and €1.7 billion.
A leaked memo shows it was decided to proceed with the overruns because it was 'the least worst option'.
Minister Harris told the hearing it was a difficult decision to press ahead despite the "massive" costs, and that an external review has been ordered to examine the overspend.
He acknowledged that the overspend represents a "very, very significant" amount of taxpayers' money, and pledged to get to the bottom of what has happened.
He did dispute suggestions that the overrun has reached €1 billion, suggesting the actual figure is still a "colossal" €450 million.
He pointed out that lower figures were based on much earlier versions of the project, and that comparisons should be made to an estimate brought to Government in 2017.
Plan for the new children's hospital in Dublin
Minister Harris said he had to consider three options when the overrun became apparent - to pause the project; to re-tender the project; or to proceed.
Explaining why he went with the third option, the Health Minister said: "It was one of the hardest decisions that the Government's made in my time in Government - certainly the hardest decision I've asked the Government to make.
"If this was any project, you might be saying 'hang on a second, we can't go any further with this'. But we couldn't actually stop this project, because it's so badly needed in terms of the infrastructure for our children."
He stressed: "What I'm not going to do is entertain any talk of pausing the project, moving the project.
"This hospital is being built.. it's underway, the children of Ireland need it. Yes, it's expensive - perhaps too expensive - but we've got to get this project done."
The new National Children's Hospital is being built on the campus of St James's Hospital in Dublin, with the facility expected to open in 2022.
