But why bother with any of the above plans for converting the Charlemont to Sandyford section into a metro at all?
The only way to actually use the available capacity between Charlemont and Sandford involves stupid stuff like 55m trams and adding turn-back facilities near Ranelagh which just means dumping people who want to get to the centre kms from their desired destination.
The fix is simple and obvious - connect the segregated metro-like section to the end of the METRO line that is being built and run it as a metro. Extend the on-street tram system south from Harcourt St. to wherever you want and run it as a tram line.
Build a proper north/south metro spine connecting high density and high employment centres and build the Interconnector to provide east/west and heavy rail interconnectivity. Everything else is not only a distraction - it's a direct hindrance to developing PT in Dublin.