You can see the cranes as you drive up Dublin’s South Circular Road. Six of them in a crop perched on a natural incline.

Three more tower cranes will be installed on the site of the National Children’s Hospital as it rises from the massive crater dug in front of St James’s Hospital.

There has been much controversy over the location of what is one of the biggest investments in paediatric care ever undertaken in this country.

The entity charged with the delivery of the project, the National Paediatric Hospital Development Board, has seen four chairmen come and go through various scrapes along the way.

The current €450m cost overrun is, however, more than just a scrape.

As noted in the Mazars report, commissioned by the board in the wake of its escalating cost crisis and obtained by RTÉ’s Prime Time: "It has exposed the project to significant reputational risk."

Prime Time has pieced together the story of what happened behind the scenes of the National Children’s Hospital from confidential memos and minutes of Board meetings.

It looks in detail at the contract used to tender for the job and asks if the final bill could rise beyond the current €1.7bn.

The National Children’s Hospital was almost built on a site at the Mater Hospital, on the northside of Dublin.

Some will remember the massive spacecraft of a building proposed at planning before it was turned down by An Bord Pleanála.

Model of the Mater Hospital

Sites on the M50, Connolly Hospital in Blanchardstown, the Coombe Maternity Hospital and Beaumont were all thrashed out and championed. Some campaigners still argue that it should be moved to another location.

Given the years of controversy over the location of the hospital and given that another design went all the way to detailed planning, you might think that when the time came to build this one the project would be well prepared.

And yes, if you look up the drawings submitted to An Bord Pleanála, there are a lot of them and they are in extensive detail.

But the design - and therefore its estimated cost - was not complete nor were the discussions concluded between the design team and the medical professionals who will end up working in the hospital.

This, industry professionals say, is normal up to a point on a complex build.

However, what is not normal is for such disparity to open up between what the hospital board believed the ballpark cost of the project would be and what the construction companies tendering for the job estimated it would cost.

That was the first red flag.

As the detail on the design was further thrashed out, an even bigger gap opened between the board and the contractors. It all happened under a tender contract that departed from the norm for public infrastructure projects.

The satellite centre at Connolly Hospital

The National Children’s Hospital project also includes satellite centres at Connolly Hospital in Blanchardstown and at Tallaght Hospital.

Prime Time was given permission to film at the main construction site at St James’s as part of our report. It is the first time you get an idea of what is going on in arguably the most controversial site in the country.

On the day we filmed, more than 400 construction workers were busy fixing steel and pouring concrete. The base slabs and underground car park floors are already set in parts of the site.

Our footage will relieve some that the hospital is finally being built and no doubt frustrate others who believe it should be built somewhere else. But it does show clearly this project, for all its faults, is very much up and running.

How much it will end up costing when it finally opens its doors is another matter.

Watch the full programme tonight on Prime Time on RTÉ One from 9.35pm.