Are we really certain of this? For example Cleary's recently closed but it was the only major department store in Dublin centre that didn't have either direct access from its own car park (e.g. Arnott's, BT's) or access from a linked multi-storey (e.g. Debeneham's from the ILAC or M&S from the Jervis Centre). There is a car park close by to Cleary's, i.e. Q-Park in Marlborough St, but it was difficult to get to, particularly if you were driving from the affluent Southside. Now Cleary's may have gone bust for a variety reasons but you can't ignore relative competitive disadvantage, from a lack of car parking comparable to that provided by equivalent city centre department stores, as a contributory factor.
The current proposals appear to be just a land grab by a state organization (i.e. the NTA), to the benefit of other state organizations (i.e. "public" – that is to say - state owned transportation). So you have state organizations looking after their own without any consideration of how this might affect wealth producing and service providing enterprises in Dublin.