Thanks for your answer George. I have read the OPW (Office of Public Works, Ireland) statement on Twitter - thanks to jonmor too for posting that link. It is quite hard to read unless you log into Twitter so I have reproduced their response below:

Office of Public Works, Ireland - Comments regarding the Roof Box to the Newgrange Monument.
Following the excavations at Newgrange in the 1960s, the monument’s entrance, including the roof box, passageway were fully re-instated. The suggestion made that the roof box is “50 year old” construct is entirely untrue.
Dr Robert Hensey, an archaeologist who has researched the excavation and re-instatement works carried out by Professor Michael O’Kelly confirms this position. Dr Hensey has stated, “The roof-box is attested in antiquarian accounts and drawings, in early 20th century photography, and then extensively in the substantial O’Kelly excavation archive. There is no question that the roof box is a modern construct or invention; indeed the O’Kelly archive allows us to carefully compare the roof box before and after restoration”.
The excavation and reinstatement carried out by Professor Michael O’Kelly were fully recorded and include detailed drawings and photographic images. Professor O’Kelly archives are fully available to consult upon request from the National Monuments Service Archives, Dept of Arts, Heritage, Regional, Rural and Gaeltacht Affairs

I admit I am puzzled about all this as the Irish Times describes Michael Gibbons as "an independent archaeologist based in Connemara, a former co-director of the OPW National Sites and Monuments Record Office." Lots of questions but unfortunately don't know who would be willing to answer them.