The geometry of the streets is not being used to it's most logical potential, best exemplified by the awkward traffic arrangements around St. Stephen's Green North.
With the possibility of trams arriving into Dame Street, the logical result is for Dame Street to become one way. South Great Georges Street would need to become one way outbound, to prevent the influx of traffic into an area that would not be able to cope. The city central area would need more outbound capacity than inbound, to deter the influx of traffic, and assist the exodus. Traffic bound for the city car parks would still have access, but not at the loss of priority of public transport.
Dublin Bus for their part would need to terminate less buses in the central area, and have less cross city buses, with those cross city buses that remain better coordinated to integrate with other services terminating away from the immediate central area.
Here is a map detailing the most logical traffic flow lines, allowing for one way traffic on Dame Street, to facilitate trams, and keeping the O'Connell Street, Westmoreland Street and College Green axis free from traffic, save perhaps for tour buses, and perhaps for one dedicated single deck central cross city bus service. I would see bus only access between Dawson Street, Suffolk Street and Church Lane.
There is no need at all for a bridge between Marlborough Street and Hawkins Street. I would be against it. Buses should not be encouraged to use that as a cross city artery, it is unsuitable for trams, and would make a mess of the scape of the Liffey and the environment in that prime area.