The RTE Radio Centre is only two floors, one about ground and the one below ground - a perfect simple way to have a radio centre, and not the badly mixed bag of studios the New Broadcasting House has in London. Shame when they built the new extension, they never considered simplicity in the form of structure, as it does seems the radio studios are all mixed through in that vast complex.
New Broadcasting House is hardly an 'extension', it was a totally new building which incorporated most of an older one (even old BH was mostly rebuilt)
The structure is fairly simple and flexible - above the TV studios, apps rooms and plant in the lower floors it's all open plan.
The radio studios are spread throughout the building because the departments that use them are too. Makes no sense to have the studios for Radio 3 and the studios for Radio 1 in the same place when they're built for very different programming and the production teams work in different parts of the building.