It might actually cost less to part tunnel a pair of extra lines through the city centre which could allow intercity expresses overtake commuter traffic, than CPO all that high value property. And doing that just for Wexford and Belfast trains just doesn't seem to make economic sense.
The money might be better spent bringing the dart under Heuston station, or cost less to build a large mainline through station where the Heuston line intersects the M50, with vast inexpensive car parking. Than rush hour access by road to intercity rail network becomes viable unlike now for most in the GDR. It can take well over an hour to get to Heuston by road at rush hour, and you'd be over half way down any of the motorways to Cork, Limerick, Waterford or Galway by then. The proposed park tunnel won't work because of the time it would take to get from north side or south side dart network to platform 5 at Heuston.