Walk to the bus stop, get on a bus to O'Connell Street and then get a 7 or 45 out to Ballsbridge.
There is no best way to actually walk all the way unless you like walking an awful lot and like taking a wiggly route through the city centre. It would take you something well over an hour for the walk and that's assuming you aren't checking a map along the way.
Have a look at the trip planner on and stick in your start and end point to see what kind of route they recommend.
z