@MagicFox - I think that's what lots of people are worrying about - which is understandable given the threats that have been made recently, but this was started a number of years ago, well before those threats were made.
I'm sure it could double as that kind of functionality, but it sounds more like it was intended to be a kind of HQ where they process intelligence from European espionage activities. This might be located underground for security reasons - no signal, can't listen, can't view from above, can't monitor activity etc. Most sites used for handling high level intelligence and security information will have these kinds of spaces - maybe not underground, but no windows, thick walls, signal blocking capabilities etc.
Obviously it's concerning in itself that they're evidently running those kinds of operations from Ireland, but it's not necessarily an indication of nuclear intent in itself.